3 x13: 9‘ g «A 4 Aerial Photographs in Geologic Interpretation * and Mapping 3 f / ‘~. ’i'fl.) ”GEOLOGICAL SURVEHI/ EROFESSIONAL PAPER 373 — Aerial Photographs in Geologic Interpretation and Mapping By RICHARD G. RAY GEOLOGICAL SURVEY PROFESSIONAL PAPER 373 Tne me of oerialpnotograp/zs to oomin ondlimtive and ondntimtive geologic information, and inytrument procedures employed in compiling geologic data from aerial p/zotogrdpns UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1960 UNITED STATES DEPARTMENT OF THE INTERIOR FRED A. SEATON, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director The US. Geological Survey Library has cataloged this publication as follows: Ray, Richard Godfrey, 192m Aerial photographs in geol ping. Washington, US. Gov ogic interpretation and map- t. Print. 03., 1960. vi, 230 p. illus., maps, diagrs. 30 cm. (U.S, Geological Survey. Professional paper (373) Bibliography: p. 224-227. 1. Aerial photography. 2. Photographic surveying. 3. Geology- Maps. (Series) For sale by the Superintendent of Documents, US. Government Printing Office Washington ’25, DC. — Price $2, (paper cover) ‘ um SCIENCES LIBRARY CONTENTS Abstract ___________________________________________ Introduction _______________________________________ The aerial photograph ___________________________ Factors that affect the photographic image ________ Focal length and flying height ________________ Film and filter combinations _________________ Lens angle _______________________________ - _ Viewing of photographs- -- - - - - - ,,,,,, - - ,,,,,,,, - - Interpretation ____________________________________ _ _ Interpretation factors _______ - - ,,,,,, - - - - ________ Recognition elements ______________________ _ _ Photographic tone ,,,,,,,,,, - - ,,,,,,,, - - Color _____________________________ - - - - Texture _____________________________ - _ Pattern ________________________________ Relation to associated features ___________ Shape _________________________________ Size ___________________________________ Combinations of recognition elements _____ Vertical exaggeration ______________________ - _ Scale of photographs ______________________ - _ Photogeology in geologic mapping ________________ Kinds and amounts of information ______________ - _ Lithologic character of rocks _________________ Sedimentary rocks ______________________ Igneous rocks __________________________ Metamorphic rocks _____________________ Structure __________________________ - - - - - - - _ Flat-lying beds _________________________ Dipping beds ___________________________ Folds __________________________________ Faults _________________________________ Joints __________________________________ Cleavage and foliation ___________________ Unconformities _________________________ Interpretation of aerial photographs in petroleum geology __________________________________ - _ - - Folds_-_-- - _ - _ - - _. Bedding ___________________________ - - _ - Drainage ______________________________ Topography ________ - Sofls eeeee --- ----- ,,,,,,,,,,,,,,,,,,,, Faults ______ - ______________________________ Facies------------_----------_--_-_---_---- Regional studies ____________________________ Interpretation of aerial photographs in search for ore deposits ,,,,, - ____________________________ Structural guides- __________________________ Lithologic guides ___________________________ Physiographic guides ________________________ Botanical guides ____________________________ 905 *U a: wwmmmamggfimNNHHug NHD—‘r—ID—‘F—‘t—Ih—lr—Il—‘D—‘t—IF-‘D—ID—‘r—ID—I r—cocowwooxlcuczmnuewwwwi— 23 24 24 24 26 26 26 27 29 29 30 3.0 30 30 ‘) u 31 32 Interpretation—Continued Interpretation of aerial photographs in engineering geology ______________________________________ Surficial materials ___________________________ Ground conditions ______________________ Elements of soil pattern _____________ Landform ______________________ Drainage characteristics _________ Erosional characteristics _________ Photographic tone ______________ Color __________________________ Vegetation _____________________ Land use ______________________ Permafrost _________________________ Distribution and type of vegeta- tion _________________________ Polygonal relief patterns _________ Pingos _________________________ Features resulting from thawing- - Absence of permafrost ___________ Erosion, transportation, and deposition--_ - Landslides _________________________ Beach erosion ______________________ Bedrock ___________________________________ Geologic structure and type of rock _______ Interpretation of aerial photographs in hydrologic studies ______________________________________ Locating potential ground-water sources _______ Water-loss studies __________________________ Other applications __________________________ Instrumentation ____________________________________ Kinds of instruments ____________________________ Measuring devices for use with paper prints--- - Plotting devices for use with paper prints ------ Measuring and plotting devices for use with paper prints ------------------------------ Measuring and plotting devices for use with glass- plate diapositives ------------------------- Other instruments -------------------------- Exaggerated-profile plotter --------------- Interval-measuring device ---------------- Universal tracing table ------------------ Measurement ---------------------------------- Principles of vertical measurement ------------ Determination of altitude differences by the parallax method ---------------------- Use of stereometer-type instruments-- Use of double-projection instruments-- Tilt _______________________________ Determination of altitude differences from a single photograph --------------------- III Page 33 33 33 34 34 34 35 35 36 36 37 37 40 40 40 40 41 41 41 42 45 45 48 48 48 48 48 48 50 51 54 55 56 IV Instrumentation-Continued Measurement#Continued Geologic uses of parallax measurements ________ Determining strikes and dips _____________ Determining stratigraphic thickness _______ Isopach mapping ___________________ Facies change, _ ____________________ Structure contouring ____________________ Notom-15 quadrangle ________________ Discovery anticline _________________ Determining displacements on faults ______ Determining stream gradients ____________ Direct determination of slope ________________ Dipping platen, ________________________ Stereo slope comparator _________________ Dip estimation-” - _ - , ,,,,,,,,,,,,,,,,,, Other methods of determining angles of slopes _______________________________ Other methods for determining quantitative data- CONTENTS Page 56 56 57 58 59 60 60 61 61 63 63 63 63 64 64 64 Instrumentation——Continued Plotting _______________________________________ Construction of control layout ________________ Orthographic positioning _____________________ Plotting on orthographic base maps or con- trol layouts __________________________ Plotting on orthophotographs ____________ Plotting on gridded base maps ___________ Vertical positioning in cross sections ___________ Interpretation, measuring, and plotting systems- _ _ 1 Instrument capability _______________________ Relation between instrument capability, scale, and vertical measurements _________________ Horizontal positioning _______________________ Source and identifying data of aerial photographs _______ References cited ____________________________________ Index ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, ILLUSTRATIONS FIGURE . Geometry of the vertical aerial photograph . Spectral reflectance curves of weathered samples of Bernal formation and San Andres limestone _____________ . Drainage map of large area in vicinity of ring dike shown in figure 54 ____________________________________ . Generalized soil map of area shown in figure 48 . Sketch showing long tributary streams on down dip side of strike valley in area of low dips _____________________ dip side of strike valley in area of steep dips ________________ . Drainage map of area shown in figure 74 and vicinity __________________________________________________ . Drainage map of area shown on left half of figure 73 ___________________________________________________ 9. Drainage map of area shown in figure 94 _____________________________________________________________ 10. Stereometers ______________________________________________________________________________________ 11. Stereo slope meter ______________________________________________________ 12. Parallax ladder _________________________________________________________ 18. Kelsh plotter _______________________________________________________ 1 2 3 4 5. Sketch showing long tributary streams on up 6 7 8 19. Exaggerated-profile plotter (lever and fulcrum type) ____________________________________________________ 20. Exaggerated—profile plotter (pantograph type) 21. Interval measuring instrument ______________________________________________________________________ 22. Universal tracing table ______________________________________________________________________________ 23. Diagram showing relation between absolute stereoscopic parallaxes and horizontal distances actually measured with stereometer-type instruments in determining differences in altitude from paper prints __________________ 24. Diagram showing relation between absolute stereoscopic parallaxes and vertical distance measured with double- projection type instruments in determining 25. Sketch showing correct orientation of photo 26. Diagram of gently dipping beds showing re graphs differences in altitude from glass plates ________________________ for stereoscopic viewing ________________________________ lation of stratigraphic thickness to differential parallax determined at any two points along dip direction and at the formation contacts ______________________________________ 27. Diagram of gently dipping beds showing relation of stratigraphic thickness to differential parallax determined by floating—line method ___________________________________________________________________________ 28. Diagram of steeply dipping beds showing relation between top and bottom of bed ____________________________________________________________________ 29- Diagram Showing graphic reconstruction of marker bed along line of section _________________________________ 30. Diagram showing graphic construction in positioning structure-contour lines on top of marker bed and the projecting of these lines to their relative horizontal positions along the line of section ______________________ 31. Diagram showing relation of structure—contour positions along two lines of section at different altitudes ......... 32. Dipping platen ____________________________________________________________________________________ 33. Stereo slope comparator ____________________________________________________________________________ Page 66 67 67 67 69 69 69 69 72 74 74 222 224 229 Page 10 12 20 20 22 25 28 43 43 43 44 44 44 46 46 46 49 49 49 49 50 51 52 57 58 59 61 62 62 63 63 i CONTENTS V Page FIGU“ 34_ Slope conversion chart showing relation of exaggeration factor, exaggerated slope, and true Slope_ __ , _ __________ 65 35. Diagram, based on tangent functions, showing relation of true dips to exaggerated dips seen in stereoscopic models ————————————————————————————————————————————————————————————————————————— 7 — __________________ 66 36. Photogrammetric systems for geologists ...... ............................ -_ _______________________ 7o 37. Photogrammetric measuring and plotting systems for geologists showing relation of different instruments to vertical_measurment and horizontal-positioning eI‘I‘OI'S """""""""""""""""""""""""""""""" 71 38. Diagram showing datum—curvature relation to maximum expected error in measuring vertical intervals from leveled double-projection stereoscopic model- _ _______________________________________________________ 73 39. Poorly resistant pyroclastic rocks overlain by resistant capping formation (Colorado) (stereoscopic pair)1_____ 77 40. Area underlain predominantly by gently folded sandstone, shale, conglomerate, and graywacke (northern Alaska) (stereoscopic pair) ________________________________________________________________________ 78 41, Extrusive volcanic rocks and associated cinder cone (Utah) (stereoscopic pair)- _ __________________________ g _ 79 42_ Flat—lying sandstone (southern Utah) (stereoscopic pair) —————————————————————————————————————— ~— .......... 80 43. Sandstone and conglomerate beds intruded by diorite porphyry laccolith (Utah) (stereoscopic pair) __________ 81 44. Gently dipping beds of sandstone, siltstone, and conglomerate that locally contain channel-r111 deposits (Arizona) (stereoscopic pair) _______________________________________________ - _ - - g ___________________________ 82 45. Gneiss, schist, and granite flanked by dipping sedimentary rocks truncated by pediment gravel (Colorado) (stereoscopic pair) ___________________________________________________ _ > __________________________ 83 46. Landslide area (Colorado) (stereoscopic pair) ____________________________________ __ ____________________ 84 47. Orthophotograph and perspective photograph of same area ______________________________________________ 85 48. Area showing relation of residual soils to underlying rock types and structure (South Carolina) (stereoscopic pair) ___________________________________________________________________________________________ 86 49. Shale and thin sandstone interbeds capped by poorly consolidated gravel and sand (Texas) (stereoscopic pair)- _ 88 50. Recently faulted strand lines of former lake (Nevada) (stereoscopic pair) ______________________ e _- _ e _- _ e _ l , 90 51. Steeply dipping sedimentary rocks intruded by quartz diorite (California) (stereoscopic pair) ________________ 92 52. Area showing drainage characteristics in surficial materials (Wyoming) (stereoscopic pair) _______ _ ._e_e___‘_ , _ 94 53. Area of basalt flows (Idaho) (stereoscopic pair) ______________________________________________ ,_ ________ 96 54. Ring-dike area (North Carolina) (stereoscopic pair) ________ _ - - _ ________ - _ - _ __________ _ _ _ _ _ _ ____________ 98 55. Area in Maine showing esker landform on photographs of two different scales (stereoscopic pairs)___ __- _ ____ 100 56. Stabilized sand dunes (central Alaska) (stereoscopic pair) _______________________________________ _ _ ______ 102 57. Morainal deposits of two glaciations (central Alaska) (stereoscopic pair) __________________________________ 104 58. Morainal deposits of continental glaciation (South Dakota) (stereoscopic pair) ______________________ _ __-e_ 106 59. Gently dipping sedimentary rocks lying unconformably on gneiss-granite-schist complex (Utah) (stereoscopic pair) _____________________________________________________________________________ ___--_ ________ 108 60. Gently dipping sedimentary rocks out by numerous near-vertical faults (Utah) (stereoscopic pair) ____________ 110 61. Flat-lying beds of sandstone, limestone, and shale (Texas) (stereoscopic pair) ______________________ __ ______ 112 62. Area of faulted gently dipping sedimentary rocks (Utah) (stereoscopic pair) _______________ _ _____ ________ _ _ 114 63. Shale in semiarid climatic area (Utah) (stereoscopic pair) _____________ _ __ ___,_ _-______ __e_ __________ "n 116 64. Shale and limestone in humid climatic area (Virginia) (stereoscopic pair) _______ , __ ____________ _-__ ______ 118 65. Area of gently dipping limestone and shale (Pennsylvania) (stereoscopic pair) _________ _ _ _ _ - _ e _ ________ , _ _ V 120 66. Area mantled by glacial deposits (South Dakota) (stereoscopic pair) _______________ __ ____________________ 122 67. Glaciated area in southeastern Alaska (stereoscopic pair) _______________________________________________ 124 68. Approximately flat-lying limestone and interbedded sandstone and marlstone beds covered locally by alluvium and windblown deposits (Texas) (stereoscopic pair)____ ______________ ,_ ______________________________ 126 69. Area of basalt flows (Idaho) (stereoscopic pair) ______ , _ __________ , _ ______________________ , - ____________ 128 70. Area of granite (South Dakota) (stereoscopic pair) _________ , _ __________ _ - a - , a - 1 , - ______________________ 130 71. Area of granitic intrusive rocks (Wyoming) (stereoscopic pair) _____ A ____________________________________ 132 72. Area showing vegetation differences in igneous and metamorphic terrain (southeastern Alaska) (stereoscopic pair) _____________ _-,_ _______________________ "1 ________________________________________________ 134 73. Area underlain by silicic volcanic tufl’s and flows and bedded argillite or slate (North Carolina) (stereoscopic pair) _______________________________________________________________ V e , _______________ , _ ________ 136 74. Area of folded, faulted, and highly metamorphosed rocks (North Carolina) (stereoscopic pair) ______________ 138 75. Area underlain by phyllite and slightly metamorphosed slaty rocks (Alabama) (stereoscopic pair)_,_ ________ 140 76. Heavily vegetated terrain underlain by metamorphic 'and extrusive volcanic rocks (southeastern Alaska) (stereoscopic pair)” ,_-,- ____________ N ,,,,,,,, e, ____________ ,_ ______________ e-" ________________ 142 77. Flat-lying beds of limestone and shale (Kansas) (stereoscopic pair) ______ , - , _ , - , _ , - ______________________ 144 78. Flat-lying beds of sandstone, limestone, and shale (Texas) (stereoscopic pair)- , , __________________________ 146 79. Area of sparse outcrops underlain largely by gently folded sandstone, conglomerate, and shale (northern Alaska) (stereoscopic pair) _____________________________________________ ,_ ________________________________ 148 80. Gently to steeply dipping sedimentary rocks overlying schist—gneiss-granite complex (Wyoming) (stereoscopic triplet)- _____________ H, ,,,,,,,,,,,,, H, ,,,,,,,,,, H ,,,,,, ,_ ,,,,,,,, H ,,,,,,,,,,,,,,,,,,,,, ,__ 150 81. Gently folded sedimentary rocks (central Utah) (stereoscopic pair) _____ _ , - , - - - , . - _ , , , .................... 152 82. Ring dike in New Hampshire (stereoscopic pair) _____________________________________________ , - ________ 154 83. Gently dipping sedimentary rocks offset by near-vertical fault (Utah) (stereoscopic pair) ___________________ 156 — —————" , VI CONTENTS FIGURE 84. Faulted, gently dipping sedimentary rocks (Utah) (stereoscopic pair) __________ 7 ,,,,,,,,,,,,, 77 ,,,,,,,,,,, 158 85. Vegetated terrain underlain by igneous and metamorphic rocks locally covered by basalt flows (southeastern Alaska) (stereoscopicpair) ////// 77 77777777 xxxxxx 77 zzzzzzz 77777777 "77777777 ________ 7 ....... 160 86. Area Of crystalline rocks (Wyoming (stereoscopic pair) 7777 7 7 ,,,,,,, 7 7 7 7 7 7 7 7 7 7 ,,,,,,,,,,,, 7 7 _________ 7__ 162 87. Lava flow lying unconformably on gently dipping sedimentary rocks (New Mexico) (stereoscopic pair).. 7 7 164 88, Gently dipping sedimentary rocks overlain in part by river gravels (Utah) (stereoscopic pair) ,,,,,, 7 77 ,,,,, 166 89. Gently folded sedimentary rocks in heavily vegetated area (northern Guatemala) (stereoscopic pair) 7 7 7 7 7 7 7 7 7 168 90, Plunging anticline in gently folded rocks (southern Utah) (stereoscopic pair) 7 7 7 7 7 7 7 7 7 7 7 ,,,,,, 7 7 7 7 7 170 917 Stratigraphic section across gently folded anticline (Utah) (stereoscopic pair) 777 ,,,,,, 7 7 7 7 7 7 7 7 ,,,,,,,,,, 7 7 7 172 927 Gently dipping sedimentary rocks, on east side of San Rafael Swell (Utah) (stereoscopic pair) 7 7 777 7 7 7 7 ,,,,, 174 93. Uncontrolled mosaic of Umiat anticline area, northern Alaska, underlain by shale, sandstone, conglomerate, //////////////// 7777777777777777777 7 176 and graywacke777777777777777 /////////// 7777777777777777777 94. Subsurface structure re ected in surface drainage characteristics (Texas) (stereoscopic pair) 7 7 7 7 7 7 7 7 7 7 7 ,,,,, 178 95. Same photographs as figure 94, but reversed to show pseudoscopic efiect as an aid to interpretation (stereo— scopicpair)777777777777777 7777777777 7777777 77 77777 77777 7777777777777777 7 7 7 7 7777 ,,,,,,,,, 180 96, Uncontrolled mosaic of Wainwright area, northwestern Alaska7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 77777777 7 7 7 7 7 7 7 7 7 7 182 97. Area underlain predominantly by gently dipping sandstone and shale (northern Alaska) (stereoscopic pair) 7 7 7 184 98. Gently dipping sedimentary rocks out by high—angle faults (Nevada) (stereoscopic pair) 77777 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 186 99. Complexly folded and faulted area of greenstone, schist, limestone and marble (southeastern Alaska) (stereo- scopicpair)777777777777777 7777 77777777777777777777777 7777777777 7 77777777777777 77 7 7 77777 77 188 100. Igneous and metamorphic terrain (southeastern Alaska) (stereoscopic pair) 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 190 101. Gently tilted and faulted lava flows (Oregon) (stereoscopic pair) 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 192 102. Area of pegmatite dikes (South Dakota) (stereoscopic pair) 7 7 77777777777 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 194 103. Metamorphosed sedimentary rocks intruded by quartz monzonite (California) (stereoscopic pair)7 777777 77 7 7 7 196 104. Distinctive landform of gravel terraces along a major stream (northeastern Utah) (stereoscopic pair) 77777 7 7 7 198 105. Areas showing drainage characteristics in surficial material that is predominantly loess (stereoscopic pairs) 77 200 106. Area of surficial deposits, primarily Windblown (central Nebraska) (stereoscopic pair) 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 202 107. Mottled soils of drift plain (Iowa) (stereoscopic pair) 7 7 7 7 77 77 7 7 7 77 77 7 77 77 7 7 7 7 77 77 77777 77 7 7 77 7 7 204 108. Coastal plain underlain by clay, sand, and gravel (New Jersey) (stereoscopic pair) 77 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 206 109. Polygonally patterned ground in permafrost area along major stream (northern Alaska) (stereoscopic pair) 7 7 7 208 110. Well—developed pingo in area of permafrost (northern Alaska) (stereoscopic pair) 7 7 7 7 7 .7 7 77 77777777777 7 7 77 7 210 111. Area of permafrost (northern Alaska) (stereoscopic pair) 7 7 7 7 7 7 7 7 7 7777777 7 7 77 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 77777 212 112. Conspicuous beaded drainage in area of permafrost (northern Alaska) (stereoscopic pair) 7 7 7 7 7 7 7 7 777777 77 214 113. Landslide area in volcanic and sedimentary terrain (New Mexico) (stereoscopic pair) 7 77 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 216 114. Damsite area (southeastern Alaska) (stereoscopic pair) 7 7 7 7 7 7 7 777777 7 7 7 7 218 115. Gridded photograph7 7 77777777777 7 7 7 7 7 7 7 220 116. Gridded base map77 777777 7777 777777777777 7 777777777777 7 7777777777 777 77777777 221 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING By RICHARD G. RAY ABSTRACT Aerial photographs today are widely used to obtain both qualitative and quantitative geologic information; vertical aerial photographs are used almost to the exclusion of other types. Techniques and procedures described herein relate pri- marily to vertical photography. Geologic interpretation of aerial photographs is based on the fundamental recognition elements of photographic tone, color, texture, pattern, relation of associated features, shape, and size. The scale of photographs, as well as the vertical exag- geration that is present in most stereoscopic models, also are significant in photointerpretation. The amount of geologic information that may be obtained from aerial photographs is primarily dependent on the type of terrain (igneous, metamorphic, or sedimentary), climatic environment, and stage of the geomorphic cycle. Because features are more readily recognized where strong differences exist in the erosional resistance of adjacent rocks, sedimen- tary terrain may be expected to yield the greatest amount of information from aerial photographs. Metamorphic ter- rain may yield the least information because metamorphic processes tend to destroy differences that may have existed in the unmetamorphosed rocks. Combinations of criteria such as photographic tone, texture, pattern, and vertical exaggera- tion permit inferences as to rock type and geologic structure, which are important in petroleum exploration, ore-deposits search, and engineering geology. In petroleum exploration aerial photographs provide a wealth of information primarily with regard to potential structural traps. Folds commonly may be interpreted from a study of strike and dip of bedding and from stream patterns; anom- alous stream characteristics, such as stream deflections, may suggest subsurface structures. The variety of photorecogni- tion criteria that suggests faults permits aerial photographs to be of particular use in many ore-deposits studies. Analy- sis of soil patterns yields information regarding permeability of the surficial materials that are a concern of the engineer- ing geologist. The instruments used for viewing photographs, measuring geologic features, and compiling geologic maps range from simple stereoscopes and stereometers, or measuring bars, to complex double-projection instruments such as the multiplex or Kelsh plotter. Some instruments require the use of pa— per prints; others require the use of glass—plate diapositives. Measuring devices primarily provide spot heights or differ- ences in altitudes; these quantitative data may be geologi- cally significant in measurement of stratigraphic thickness and dip of beds and in structure contouring. Accuracy of vertical measurements is related fundamentally to the scale of photography and the instruments used for making meas- urements. In general, accuracy of measurement is greater when large-scale photographs are used and when double- projection instruments rather than simple parallax bars are employed. INTRODUCTION Use of aerial photographs to obtain geologic infor- mation~popularly called photogeology—has contrib- uted to mineral and fuel discoveries, and to engineer- ing geology studies, as well as to the general geologic mapping of many areas; it also has increased the efficiency of many geologic mapping groups by adding speed, economy, and accuracy to areal mapping as well as adding certain geologic information that is impos- sible, difficult, or economically impractical to obtain by routine field~mapping methods. The past financial success of consultants in geologic interpretation of aerial photographs attests to successful geologic appli- cation of photogeologic procedures and techniques, particularly in petroleum geology, and to lesser extent in mining and engineering geology. Many technical papers on the geologic interpretation of aerial photographs have been published, primarily since the end of “’orld War II; on several occasions symposia on photointerpretation, and specifically pho— togeology, have been presented. Much of the informa— tion currently available has been published as indi- vidual articles and they are scattered throughout the literature; comprehensive treatments on the geologic interpretation of aerial photographs are generally un— available now (1959) (see Eardley, A. J., 1942; Smith. H. T. 7.19431); and Petrusevich, M. N., 1954). In addition to the published information much unpub- lished information also has accumulated in recent years. The collection and synthesis of much of this information. together with discussion of photointer- pretive and related photogrammetric procedures, are the main objectives of this paper. The paper is par- ticularly intended as a guide to geologic interpretation of aerial photographs as applied as a tool in recon— naissance geologic mapping, and as a reference to pub— lished papers representative of the photogeologic literature. 2 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING The primary objectives of photogeology are (a) to contribute to geologic mapping, which in turn is basic to mineral and fuel exploration, engineering geology studies, some water-resources investigations, and re- lated studies, and (b) to contribute to geologic knowl- edge through research. The objectives of photogeology may be economic or academic, and procedures used can be expected to differ widely, ranging, for example, from use solely of simple lens stereoscopes for quali— tative interpretation to employment of precision stereo- plotters for making geologic measurements. Regard- less of procedures used, however, photogeology to date has contributed principally to the broad aspects of geologic study, that is, in mapping distribution of rock types and structures. The interpreter can only infer the composition of rock types from photographs; he cannot identify mineral types or absolute ages of rocks. Thus, maximum use of photogeologic proce- dures is attained by combining photogeologic studies with various laboratory studies and with field investi- gations. The uses of aerial photographs and the various pho— togeologic procedures are discussed herein under the general headings of interpretation and instrumenta- tion. The part dealing with interpretation is widely illustrated with stereoscopic pairs and triplets, single photographs, and mosaics, together with explanatory notes, to demonstrate the kinds and amounts of geo- logic information that can be obtained from aerial photographs of differing geologic terrains and to illus- trate some uses of photographs in specific studies, such as the search for petroleum. These photographs dem- onstrate primarily the qualitative uses of the funda— mental recognition elements in geologic interpretation; they represent only a sampling of the tremendous number of illustrations available. Because reproduc— tion processes tend to obscure details only subtly ex- pressed on aerial photographs, selection of photographs used in this paper was confined principally to those that illustrate clearly defined recognition elements and geologic features. All aerial photographs (figs. 39— 114) are at the end of the paper. Agencies holding the negatives of these photographs are given in a list that follows the photographs. Methods and significance of determining quantita- tive geologic information from aerial photographs are described under the heading of instrumentation. THE AERIAL PHOTOGRAPH The aerial photograph is an instantaneous record of the ground details as determined chiefly by the focal length of the camera lens, the flying height of the airplane at the time of exposure, and the film and filters used. It may also be defined as a composite of photographic images, which make up the recognition elements used for interpretation. The aerial photo- graph is a perspective photograph that is geometri- cally related to the type of camera in which it is taken; it may be a vertical photograph, taken with the camera axis pointing essentially vertically down, or it may be an oblique photograph, taken with the camera axis purposely tilted from vertical, generally 20° or more. Vertical photographs currently are used almost to the exclusion of oblique photographs for geologic interpretation, and most photogrammetric instruments used in photogeology in the United States are designed to accommodate vertical photographs; hence the following discussion is limited to techniques applicable to the study of vertical photography. Twin low-oblique photographs—those in which the apparent horizon is not shown generally may be transformed and the resulting paper prints used as vertical photo- graphs; however, low—oblique photographs cannot be used in many stereoplotting instruments unless the instruments are specially designed. A general knowledge of the geometry and terminol— ogy used with vertical aerial photographs is necessary if one is to understand and make maximum use of photographs for interpretation and mapping purposes. The terminology and geometry of the vertical photo- graph are presented in figure 1. FACTORS THAT AFFECT THE PHOTOGRAPHIC IMAGE Factors that affect the photographic image, and hence interpretation, may be divided into two groups: (a) The relatively constant man—controlled factors, such as focal length of lens, flying height, film and filter combinations, and lens angle; and (b) the vari— able natural factors including color of objects photo- graphed, position of an object with respect to the angle of sun, amount of haze in the atmosphere, and others. The constant factors are discussed briefly be- low; effects of natural factors on the photographic image are described where appropriate throughout the text. FOCAL LENGTH AND FLYING HEIGHT Focal length and flying height may be considered together because of the relation of photograph scale to these two factors. In terms of focal length and flying height, the average scale of a photograph is expressed as _f 3'17 or focal length (feet) , ____’_,_/ Scale flying height (feet). _7 INTRODUCTION ~’ l, owmfibdmfn—HY I I .E?Lw3oaa 23.5w 135.5,, 2: we bumecocIA @5th N LQMLMOHOLQ Eva: Erato Mcamgmogoca EUmc .9330 .CQSmowoca 302:; 3:: E tum: zamxmowoca 5:5 mmuSEoQ «Eon E9055: Ema: meal I / Iwocfiwfi mwmn t5 \ x:\ M FIGURE 9.—Drainage map of area shown in figure 94. Radial drainage over structure, A of figure 94, stands nular pattern around the structure. Stream deflections, as out on the drainage map which is unobscured by the at D in figure 94, are commonly more readily observed on abundant details of the photograph. Note that the radially the separate drainage map or in pseudoscopic View of the draining streams flow into larger streams that have an an- model (see fig. 95). INTERPRETATION OF AERIAL PHOTOGRAPHS IN PETROLEUM GEOLOGY structural trend. Normal to this trend other elongate hilltops sloped in the direction of the inferred dip of beds. Also, streams flowing in the dip direction had a slightly less well developed dendritic pattern than those flowing opposite to the dip direction. There also appeared to be a correlation between slopes of the tops of cut banks and the direction of dip. Plotting of all such data gave a consistent apparent structural pattern ——a plunging anticline—of unknown reliability. A subsequent seismic survey corrobrated the photogeo— logic interpretation. SOILS In some areas devoid of outcrops, soil patterns or soil distributions may be useful in delineating or sug— gesting structures. Both Woolnough (1934b, p. 221, 224) and Christensen (1956, p. 858—859) cited exam- ples of the usefulness of aerial photographs in this regard. Wheeler and Smith (1952, p. 106) stated that contrasts in soil tone accentuate unusual drainage rela— tions on the northern flank of a structure in the south- west Maysville area, Oklahoma; and in southwestern Alabama differences in cultivation pattern reflect relief and soil characteristics by which geologic units of Tertiary age beneath younger unconsolidated materials can be differentiated. DeBlieux and Shepherd (1951, p. 98) described tonal and textural anomalies in west-central Texas that resulted from differences in soil types, soil—mois- ture content, and erosional characteristics. They cited an anomaly at Salt Fork on the Brazo River where “* * * dark and coarsely hummocky surface strongly contrasts with the lighter color and finer texture of the surrounding surface * * *” (DeBlieux and Shep- herd, 1951, p. 98). The anomaly was strikingly simi— lar to that of a reef producer in Scurry County. De- Blieux and Shephard (1951, p. 88) emphasized that how a structure forms is unimportant in a study of tonal, textural and other anomalies, because the surface evidence is identical, whether the structure has resulted from upward movement, differential downwarping, reef or residual topography, or combination of these features; it is important initially that a surface anom- aly can be recognized. FAULTS ,Faults are significant in the geologic interpretation of petroleum areas because they may be deep seated and of major importance in localizing oil accumula- tion. Where faults are genetically related to structures that may be oil bearing the distribution pattern of faults may be of considerable use for interpreting the locations of those structures. The many surface ex— pressions of faults are described above (p. 21). Desjardins (1952, p. 82) stated that surface fault 29 and fracture lines, expressed as lineations on erial photographs, form patterns that bear a definite rela— tion to deep—seated salt domes and to subsurface aults of the Gulf Coastal Plain. Disturbance center , be- lieved to be underlain at depth by salt domes, m y be indicated by the fault patterns, shown on photogfaphs as “Sharply curving lines which are strikingly con— centric” or as “Prominent transverse lines radi ting usually from some marginal point rather than from the center of the structure” (Desjardins, 1952, p 82). Desjardins believed that such fault patterns we the result of continuous application of small stresses 0 used by rising salt masses rather than an accumulatiin of stresses that would produce large amounts of ove- ment; he also felt that faults probably would form in interdome areas as well as over the domes. The genetic significance of the fault pattern is thus 0 'tical if distribution of faults is used as a basis for loc ting the dome itself. Desjardins (1952, p. 83) mide a strong distinction between surface expressions of fEults, which he associated with deep-seated domes, and ther criteria, such as topographic expression, wideni g of natural levees, and looping of stream channels, hich he associated with shallow—seated domes. Although most interpretations of faults are bas d on qualitative characteristics of observed data, su h as the dislocation of correlative beds, or the contr st in light and dark photographic tones on opposite isides of a straight line, a quantitative approach to such studies may provide additional useful data. 0 the basis of azimuth—frequency studies Blanchet ((1957, p. 1748) suggested that structural and stratigraphic anomalies in some areas may be located by a statistical analysis of fractures, which he defined as the gener- ally abundant, natural lineations discernible 0n aerial photographs. The basis for analysis is a comp rison of local deviations in the statistical mean dirjction of fracture sets with regional norms for each fracture set. Regional norms are first established by pl tting fracture-azimuth frequency diagrams of the regi n in- volved. Then statistical mean directions of fr cture sets are determined by analyzing samples within ircu- lar areas centered at relatively small horizontal nter- vals of a few miles. These local mean directio s are in turn studied for local deviations which are e tered 'into empirical equations that yield structure—int nsity values. These structure-intensity values are the con- toured, to form a structure-intensity map, whic may reflect structural anomaly. The structure-int nsity map is supported by a fracture-incidence map, pre— pared in contour form, and by an analysis of dra'nage characteristics of the area in question. Blanch t ap— plied this method to a study of petroleum structures. 30 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING FACIES Although the widest application of aerial photo- graphs in petroleum geology has been with regard to structural traps, a study of aerial photographs may contribute to the location of stratigraphic traps, par- ticularly if photogrammetric measurements of forma- tion thicknesses can be made at many points within the streoscopic model (see p. 59—60). From a quali— tative standpoint it can be seen in figure 97 that a rapid loss of sandy section can sometimes be detected on aerial photographs. Such “shaling” or loss of sandy section due to facies change may be important in the search for stratigraphic traps and in some areas it may be measured photogrammetrically. In other areas well-defined formational units may thin or thicken without facies change, but this also may be significant in regional studies with regard to strati— graphic traps (Christensen, 1956, p. 861, fig. 6). Meas- urements of stratigraphic sections needed in such stud- ies commonly can be made from aerial photographs. REGIONAL STUDIES The significance of aerial photographs in regional petroleum studies is widely recognized, but very little information is present in the literature with regard to this phase of photograph use. Not only may meas— urements be made from aerial photographs with regard to strike, dip, stratigraphic thickness, and closure, but the regional view of the mosaic or small-scale photo— graphs permits a continuity of photographic criteria that may be of structural or stratigraphic significance. Overall geologic relations are brought together, and areas where detailed study is needed may be indicated. However, the limitations of the photogeologic tech- nique must be constantly borne in mind. As Wheeler and Smith (1952, p. 112) pointed out, the validity of the technique “* * * depends a good deal upon the human element and the utilization of all applicable geologic data.” They warned that “* * * the student of photogeology may * * * become overly impressed with geometric design of lines, masses, and tones so that, in the absence of understanding of the tectonic history he may find compatible alignments of stream courses, structurally inexplicable lineations or shapes and soil patterns that are not consistent with the known or expectable products of diastrophism” (1952, p. 112). An emphasis on regional study is indeed jus- tified, however, if maximum use is to be made of aerial photographs, and this is true whether photographs are used in long-range exploration programs, as in reconnaissance mapping of broad basin areas, or in the ultimate study of local structures. INTERPRETATION OF AERIAL PHOTOGRAPHS IN SEARCH FOR ORE DEPOSITS The search for ore deposits, similar to that for petroleum, requires favorable host rocks or structures, or both, and it is therefore necessary to determine the general lithologic character and distribution of rocks and associated structures as a basis for carrying out a mineral survey of an area. Aerial photographs may contribute geologic data not only with regard to the fundamental guides of lithology and structure in ore search, but they also may reveal details of physiog— raphy, which may be an important ore guide in the search for placer deposits. Botanical guides to ore bodies may also be detected on aerial photographs of some areas, although little work has been done in this field. STRUCTURAL GUIDES Where fractures have been significant in localizing ore deposits aerial photographs may reveal significant structural data relating to trends and offsets of frac— tures, even in areas where surficial cover is extensive. This is due in large part to the great variety of cri— teria suggestive of fractures (see p. 21). In many areas it is possible to determine spatial relations of sets of fractures, and to infer the relative ages of the frac— tures from the aerial photographs, although results of field study are necessary to determine the significance of the patterns. Aerial photographs are presently most useful in study of those ore deposits associated with fractures. In many mining districts where mineralized fractures form a vein pattern that is repeated throughout the district, an analysis of the fracture pattern and distri- bution may provide significant data in the search for new ore bodies. Reed (1940) described the use of aerial photographs in a mineral-deposits study of the heavily timbered Chichagof mining district of Alaska where gold-bearing quartz veins are associated with faults and shear zones (see fig. 99). There the faults are commonly expressed as depressions owing to greater erosion of soft material from fault zones (see also Twenhofel and Sainsbury, 1958, p. 1433). Reed (1940, p. 44) stated that many faults of moderate surface expression would probably have been overlooked on the ground, and that many faults of weak surface expression were seen only on aerial photographs. Joliffe (1945, p. 604), in a discussion of photogeologic prospecting in the Yellowknife area of Canada, noted that some of the largest gold-ore bodies “* * * lie in strong shear zones that are marked by rather distinc- tive gullies in relatively massive, resistant volcanic rocks.” Many of these shear zones and other linea- ments are conspicuously shown on aerial photographs. INTERPRETATION OF AERIAL PHOTOGRAPHS IN SEARCH FOR ORE DEPOSITS 31 Gross (1951) pointed out that in the glaciated Drys- den—Kenora area of northern Ontario and the Gold- fields area of northern Saskatchewan, Canada, there are many linear features seen on aerial photographs but that only certain ones represent faults. He de- vised a scheme of statistical analysis to show the like- lihood that certain linear trends were faults ( see p. 21). Concentration of prospecting along these linear fea— tures in the Goldfields area resulted in discovery of pitchblende deposits. Because faults commonly are interpreted readily from aerial photographs, photogeologic study may aid in locating extensions of faulted ore bodies. 'Faults that offset an ore body may show an orderly spatial arrangement, and information regarding direction of displacement may be extrapolated to similar faults whose displacement is masked by vegetation or sur—- ficial debris. VVillett (1940) found that aerial photo— graphs provide significant information on the location of faults cutting the Invincible Lode, a gold—quartz deposit in the Glenorchy district of New Zealand. 4 The photographs strikingly show the offset of a lode east of the Invincible Lode by a series of parallel faults, which suggest that similar conditions exist at the In- vincible Lode where the evidence is not as readily observed because of surficial debris. In the Invincible Lode area one major stream flows parallel to the strike of the faults, and a second stream is deflected along a fault for about 1,000 feet, then bends back to its old course; the direction of deflection also indicates the horizontal direction of displacement of the quartz lode. The pattern of displacement, based on photogeologic evidence together with ground evidence, suggests the location of faulted extensions of the lode. Major anticlinal and domal structures, readily inter- preted from aerial photographs of many areas on the basis of drainage characteristics or strike and dip patterns, are with notable exceptions of relatively little importance in ore search. Where folds have been a factor in localizing ore, as in some metamorphic rocks, they commonly are too small or the geology too com- plex for existing aerial photographs to provide useful guides in prospecting. LITH OLOGIC GUIDES Lithologic guides may be considered from the stand— point of the parent and host rocks, or from the stand- point of mineralized or altered zones, which are more appropriately termed mineralogic guides. Photogeo— logic interpretation of rock types (see p. 16~19) may outline broad target areas of favorable parent and host rocks (fig. 100). Lueder (1953, p. 823) suggested that various locations can be rated on the basis of favorable photogeologic evidence; the most favorable area would become the prime target area for further investigation. Where some local field criteria are avail- able, specific ore-bearing formations may be tra ed on photographs (fig. 98). At Ross Lake, Canada, the fact that rare-earth-bearing pegmatites of small areal extent were recognizable on photographs was a major factor in the discovery of certain tantalite deposits (Joliffe, 1945, p. 604). The use of color aerial photography will increase many times the amount of lithologic information that can be obtained from a photogeologic study. This additional information will contribute not. only to the reliable interpretation of host or parent rocks, but may permit local guides, such as alteration zones, to be readily recognized. In a study of the hydrothermally altered volcanic terrain near Tonopah and Goldfield, Nevada, information obtained from reversible-type color transparencies was contrasted with data obtained from thin sections of rock types. Kent (1957, p. 868) stated that “In both areas a general correspondence between coloration and alteration was observed. Zones of different coloration within a lithologic unit repre- sent different stages of alteration. Colors of rocks in early stages of alteration are useful in recognizing lithology, but highly altered rocks have the same color within several different lithologic units. The colors associated with highly altered rocks are easily recog- nized on color aerial photographs; thus their distri— bution and their relationship to structural features can be studied.” In addition Ray (1958, p. 37) wrote that “In one area of altered andesites a pale green color on color aerial photographs was found to represent an intermediate stage in the alteration resulting from partial silicification, with pale green chalcedony, and deuteric alteration and later oxidation of iron or aques. A light color in another zone was found to result from brecciation of dacite and attendant alteration to car— bonates. A rather intense orange color was indicative of highly altered zones in all localities observed. How- ever, the orange coloration did not indicate the rock type involved nor the type of alteration prese 1t, but rather was the result primarily of weathering of iron minerals following their partial breakdown by deu— teric alteration.” l PHYSIOGRAPHIC GUIDES i Some ore deposits are topographically expressed or are related to structures, rock types, or surficial de— posits that are so expressed (fig. 102). Hence, topogra- phy may be considered a guide in ore search, although it does not by itself indicate the presence of ore. Be— cause fractures commonly are topographically ex— 32 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING pressed as rectilinear depressions, a possible physio- graphic guide to ore as interpreted from aerial photo- graphs may be said to exist in many areas. Where fractures have been mineralized, and the vein filling stands in relief, topography may be a useful guide in ore search. However, many mineralized veins are poorly resistant to weathering and erosion, and occur as topographically low areas. Where topography results in a photographic texture it may be possible to delineate areas of favorable host rocks, as the paraschists in Surinam (Zonneveld and Cohen, 1952, p. 155). Grantham (1953, p. 336) pointed out that in the wet Sierra Leonne area of Africa, the “* * * sharp change in topography from linearly ar- ranged hills, controlled by schist and migmatite, con- trasts with the irregular features of structureless granite.” An analysis of the physiography of an area may provide significant information, particularly in relation to placer deposits. As many placers have a complex history, it may be necessary to work out the alluvial history of a valley or valley system for maximum in- formation in locating placer pay streaks. The arrange- ment and positions of old stream channels may be in— terpreted from meander scars, oxbow lakes, and differ- ent terrace levels that are commonly seen on aerial photographs. Photographs also may provide gradient measurements of present or past streams; this infor- mation may be significant inasmuch as placer concen— trates occur especially where stream gradients flatten. Buried stream channels, also the site of some ore de- posits, may be interpreted from photographs of some areas on the basis of topographic expression (fig. 101). In many parts of the western Sierra Nevada, Tertiary stream valleys have been filled and gold-bearing gravel buried by younger lava flows. The greater resistance of the lava flows compared with the surrounding rocks has resulted locally in sinuous cappings of volcanic rocks that stand above the present-day stream system. Table Mountain, California (Loel, 1941, p. 384, 386— 387), is an outstanding example of basalt capping auriferous gravel of a buried channel. The location of stream-channel deposits recently has received wide attention in the search for uranium de- posits on the Colorado Plateau in Western United States. There the Triassic Shinarump member of the Chinle formation is thicker in channels cut in the underlying Moenkopi formation, which is also of Trias- sic age. Some channels can be observed directly on aerial photographs (fig. 44); others may be inferred from isopach measurements, which in some areas may be obtained from aerial photographs. (See p. 58—59). The uranium is associated with carbonaceous materials, largely plant remains, scattered throughout the chan- nels. BOTANICAL GUIDES Much field and laboratory. work has been accom— plished with regard to plant distribution and plant concentration of certain elements as indicators of ore. Little has been done, however, on plant characteristics or associations that may be observed or interpreted from aerial photographs, and herein lies a field of re— search that has much potential. There appear to be three aspects of botanical study from aerial photo- graphs that may contribute to the location of ore deposits. These involve (a) color differences in vegeta— tion brought about by concentration of certain chemi- cal elements, (b) species difl'erences or absence of vege— tation owing to concentration of certain chemical elements, and (c) distribution of vegetation suggesting favorable structures or rock types. Color differences in vegetation, due to mineral and chemical concentra- tions, that may be reflected on aerial photographs have received almost. no attention, and offer an interesting field of study. Species differences or lack of vegetation has likewise received little attention in ore search, al- though Walker (1929, p. 50—51) noted that air search was made for “dambos” as indicators of copper de- posits in central Africa. Dambos are open spots in the bush cover, where vegetation does not grow because of the concentration of poisonous copper salts in the soils. Walker noted that the Roan Antelope copper deposit was marked by such a dambo, although the deposit extended beyond the limits of open area. Identification of species from color differences due to concentrations of certain chemical elements may be particularly amen- able to detection on color or other special aerial pho- tography. > Plant and tree distribution may be useful indicators of structures or of rock types favorable to ore deposi— tion; vegetation distribution may reflect physical char— acteristics of the rocks, such as porosity or presence of fractures, although some rocks, such as limestone, may be preferentially covered by a certain type of tree be- cause of chemical concentrations in those rocks (see fig. 72) rather than because of physical properties. Or vegetation may grow preferentially or trees grow higher and be alined along a fault or shear zone, which might in turn be a favorable site for ore deposition (see fig. 71). Such indirect guides in ore search contrast highly, however, with color or species differ— ences, which might suggest more strongly the presence of ore minerals. Levings and Herness (1953, p. 456—457) stated that heavy timber growth in the Corbin-Wickes area of Montana shows a preference for unaltered quartz mon— a INTERPRETATION OF AERIAL PHOTOGRAPHS IN ENGINEERING GEOLOGY zonite, which has a darker photographic tone than the surrounding andesite and dacite; ore occurs in the volcanic rocks. Zonneveld and Cohen (1952, p. 153) mention that vegetation differences permit photo- graphic difierentiation of the clay savannahs and sand savannahs of Surinam, South America. Bauxite pro- duction at Surinam’s Billeton mine (Hemphill, oral communication, 1958) is from a clay area, and hence it is important to know the distribution of such clay areas. INTERPRETATION OF AERIAL PHOTOGRAPHS IN ENGINEERING GEOLOGY Aerial photographs have been used for many years to provide topographic and land-use information in locating routes for pipelines, highways, or other rights- of—way, but only in recent years have photographs attained wide recognition as a source of geologic in— formation for engineering purposes. Many kinds of geologic information for engineering use may be ob— tained from aerial photographs. These include (a) identification and location of soil materials, particu- larly granular materials, which would provide suitable foundation sites for industrial and airport develop- ments, or highway rights-of—way; (b) identification and location of clay and clayey silt soils, which would have detrimental properties with regard to building, airport, and highway sites; (c) location of aggregate materials; (d) delineation of areas that may be under- lain by permafrost; (e) interpretation of distressed conditions in landslide areas; (f) analysis of structural geology in areas of tunnel, dam, and reservoir sites; and (g) location of sample areas for detailed investiga— tion of soil and rock materials. SURFICIAL MATERIALS Surficial materials generally are interpreted in terms of gross physical characteristics, rather than the de- tailed physical characteristics used by the pedologist. Knowledge of the gross physical properties is sufficient for many reconnaissance studies in engineering geology. Indeed, it is the grouping of several pedo- logic soil types into a few classifications that permits aerial photographs to be of significant engineering use. In the preparation of soil-engineering maps from agri— cultural reports of an area in Illinois, Thornburn (1951, p. 91) was able to reclassify 99 different pedo- logic soil types into 13 engineering groups. Lueder (1951) described the use of aerial photographs as a major tool in preparing soil-engineering maps. The inability to interpret fine details with regard to physical characteristics of the soil thus does not pre- clude the use of aerial photographs in many engineer- 33 ing geology studies. From the standpoint of en gineer- ing significance a granular material, for example, which exhibits certain diagnostic photographic char- acteristics such as coarse-textured drainage, will gen- erally react favorably to drainage, compactability, and other engineering tests, regardless of whether it is a residual granular material that has resulted primarily from weathering in place or whether it is a trans- ported gravel derived from different rock types. HOW- ever, Jenkins, Belcher, Greeg, and Woods 1946) pointed out exceptions to the general thought that photographic characteristics common to granul 1r ma- terials always signify suitable granular materi Lls for V‘ file of‘ engineering purposes. For example, the soil pr well-drained limestone has a granular texture due to aggregation of clay particles into lumps. The surface drainage may be coarse textured; but the soils break down on compaction and react as plastic soil material, and not as a granular material (Jenkins, Belcher, Greeg, and Woods, 1946, p. 80). Where pedologic data are already available they may be very useful to the engineer when interpreted in terms of engineering test data (fig. 4). Indeed, cer- tain soil mechanics tests by the engineer, such as plasticity or soil-texture tests, are similar to those used by the pedologist. Greenman (1951) suggested that the pedologic method can become the nucleus around which aerial photo interpretive and field geologic in- formation can be accumulated. But Eardley (1943, p. 567) pointed out that “In many places con litions unfavorable for the development of a normal soil, such as rugged mountains, wide sandy plains, or swampy basins may cover rather large areas. The important construction materials in such places may be better interpreted as a result of geological pr cesses than from the standpoint. of a soil classification “ * *.” GROUND CONDITIONS The interpretation of ground conditions pr )bably has received more attention in recent years than any other engineering geology application of aerial pho- tographs. Much of the literature concerns recon- naissance studies of soils in general and granular ma- terials in particular; granular materials are essential for building sites and highway and airport develop- ment (Jenkins, Belcher, Greeg, and Woods, 1946 ; and Purdue University, 1953). Special attention also has been given to photointerpretation of permafrost con— ditions in northern latitudes. With regard to highway engineering, Belcher (1945, p. 144) stated that “* * * the soil problem resolves it— self into two parts, one in which the soil in place is satisfactory as subgrade, and the second, where the soil is unsuitable. In areas of unsuitable soils some form .1 34 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING of improvement, by stabilization or insulation, is re- quired. Insulation involves the economic location of granular materials in the form of rock to be crushed, cinders (volcanic) , or sand and gravel * * *. Inasmuch as sand and gravel or other granular deposits exist to some extent in nearly every county in the country the significance of soil patterns indicating granular ma— terials is of considerable economic importance to engi- neers.” In fact, a large percentage of construction and maintenance expenses are in the costs of sand, gravel, crushed rock, and similar materials. As a tool for interpreting areas of gravel, sand, silt, and clay, aerial photographs offer an inexpensive and rapid method of reconnaissance, and according to Schultz and Cleaves (1955, p. 370) “* * * planning and exploration for large engineering projects should almost never be undertaken without a thorough study of aerial photo- graphs of the region * * *.” Photointerpretation of surficial materials must con— sider not only the surface expressions of these ma- terials but also the geologic factors reflecting the origin of these materials. A'knowledge of the origin of ma- terials permits inferences as to what kind of materials can be expected in an area, and in what locations with respect to other geologic features. Field-sampling pro— grams also could be based on the analysis of the photo- geologic data, and subsequent mineralogic and related studies by the engineering geologist could provide the soils engineer with information that would eliminate the need for many of the soil mechanics tests now performed as part of the routine laboratory analysis of all soil samples. ELEMENTS 0F SOIL PATTERN The surface expressions of granular and other soils materials have been termed collectively the “soil pat— tern.” The soil pattern consists of several elements of which the most important ones for nonpermafrost areas are considered to be landform, drainage, erosion, rela— tive photographic tone, and color; vegetation cover and land use may also be significant (Belcher, 1944, 1945, 1948; Jenkins, Belcher, Greeg, and Woods, 1946; Frost, 1946; Mollard, 1947; Hittle, 1949; and Purdue Univer— sity, 1953). Schultz and Cleaves (1955, p. 371) con- sidered that “interpretation of soils by means of aerial photographs rests largely on the interrelationships of soils and geomorphic features”; consequently landform is considered by some to be the most important ele- ment in photointerpretation of soils. Landform, how- ever, is only one element of the soil pattern and it is clearly evident that the association of several elements of the soil pattern will provide the greatest amount of information from aerial photographs. Landform Identifying the landform commonly identifies the natural process that formed it, and commonly limits the type of soils or soil characteristics that can be ex— pected. The emphasis on landform is probably valid in reconnaissance soils investigations where broad areas of several different landformsare studied and mapping is at a small scale. But where detailed engineering in— formation is needed, as in foundation studies, it is likely that the area of study will be within one land- form, mapping will be on a large scale, and other elements of the soil pattern, such as differences in rela- tive photographic tone, ,which may reflect soil— moisture content, will be of prime importance. In small—scale mapping the emphasis on landform is probably warranted particularly for constructional landforms of transported materials, such as sand dunes, river terraces, alluvial fans, and many glacial landforms, all of which have distinctive shapes, al- though not necessarily unique (see figs. 52, 55—57, and 104) ; here the transported soil materials make up the landform. In contrast residual soils associated with destructional landforms may be more difficult to inter- pret from aerial photographs, for here the parent rock makes up the landform, and the significance of soils rests in part on the subjective interpretation of rock types. However, the interpretation of other elements of the soil pattern with regard to physical characteris- tics of soils are just as valid for residual as for trans— ported materials; the engineering significance of many residual soil areas can be interpreted readily from aerial photographs on the basis of these other elements, such as drainage and erosional characteristics (figs. 63 and 65). Drainage characteristics Highly permeable soils will have good drainage re— gardless of their origin, provided that topographic positions will permit draining. In well—drained soils surface drainage normally will be coarse textured or even absent (figs. 49, 52, 66, 104, 106, and 108); in soils of low permeability drainage will normally be fine textured (fig. 63) or ponds may be numerous (figs. 57 and 66). The presence of fine- or coarse—textured drainage in surficial materials, like that in bedrock (p. 16), is primarily related to the relative re— sistance of the different materials to erosion, and hence related to permeability and grain size. Coarse ma- terials characteristically are resistant to erosion, are permeable, and have a coarse-textured drainage; fine materials commonly are less resistant to erosion, are impermeable, and have a fine-textured drainage. These relations are significant in the interpretation of engi- neering soils from aerial photographs because drain— INTERPRETATION OF AERIAL PHOTOGRAPHS IN ENGINEERING GEOLOGY 35 age texture can be readily observed. However, excep- tions to these relations may exist, as with loess, which is commonly characterized on aerial photographs by a relatively fine—textured drainage (see figs. 105 and 106) ; yet, loess in its natural environment is generally considered well drained internally. Flow or channel markings, for example on terraces, indicate that the material was deposited by running water and further suggest that the material may be granular. Frost (1946, p. 121) described flow mark— ings in an area in the midwest as consisting of light and dark streaks usually parallel to the direction of flow. The light streaks are ridges of sand or fine gravel that are probably sand and gravel bars; the dark streaks are fine clay or organic material deposited in depressions that once contained standing bodies of water. Frost (1946, p. 121) stated that “The chief significance of such m2 rkings lies in the fact that they suggest stream deposition and in order to have stream deposition at such elevated positions and occurring on * * * a large scale, there must have been large volumes of swiftly flowing water. Since swiftly flowing water allows only coarse material to settle, the terrace must contain a large percentage of gravel.” It may be de- batable that channel markings always indicate swiftly flowing water, but where some doubt exists other ele- ments of the soil pattern may reveal the character of the terrace material. In broad areas of aggradation, such as the Missis- sippi embayment, drainage characteristics revealed by aerial photographs are significant to the engineering geologist, as different soil characteristics are associated with the several features of valley alluviation. For example, backswamp areas contain clay and silty clay of high organic content, whereas natural levees consist of silt, silty sand, and some silty clay (Fisk, 1944, p. 18, 20). The levees commonly “* * * provide linear patterns of higher ground supporting distinctive vege- tation growths” (Tator, 1951, p. 717). Low areas may be swampy and also contain distinctive vegetation. Graveliferous deposits fill theold valleys and in turn are covered by various other deposits; sandbars contain permeable sand that grades downward into the gravel. The distribution of features of the flood-plain drain- age may be significant in analyzing ground conditions and locating favorable engineering materials in high- way, railroad, and airport development. Erosional characteristics Closely related to the drainage characteristics of a soil are its erosional characteristics; these are impor— tant in photointerpretation studies chiefly from the standpoint of gully erosion, which is controlled largely by the physical properties of the soil and especially by cohesion. The usefulness of gully erosion characteris- tics in photointerpretation primarily involves the shape of transverse profiles, although longitudinal profiles may provide additional information. According to re— search workers at Purdue University (1953, p. 15—16) “* * * there are three basic gully characteristics which are associated with three major soil textural groups.” Granular soils commonly develop sharp, V-shaped gullies that have short, steep gradients. Nongranular cohesive and plastic soils are generally indicated by uniform gentle gradient of gullies that extend well back into the upland, and by broadly rounded shallow V-shaped transverse profiles. The loess soils and sandy-clay soil exhibit U-shaped cross sections of gullies that have flat bottoms and low gradients. Although there may be a general adherence of gully characteristics to the major soil textural groups, ex- ceptions are not uncommon. For example combina- tions of the basic gully characteristics “* * * ozcur in ‘layered soils’ or in soils exhibiting a ‘strong profile’ ”. (Purdue University, 1953, p. 16), and compound gradients may develop. In addition to the steep or vertical slopes in deep gullies, windblown silt may de- velop silt pinnacles and catsteps or terracettes (Purdue University, 1953, p. 45). It has been pointed out (Purdue University, 1953, p. 27) that climatic efiviron- ment under which gullies form is an importa t con- sideration; for example, soft clay shale in a arid region where flash floods may occur commonly exhibit abnormally steep slopes in contrast to clay shale in areas of uniform rainfall where such slopes are usually softly rounded. Under certain conditions, such as gravel capping on clay soils in the New Jersey zoastal plain (fig. 108), gullies in the underlying clay soils may be steep and deeply incised, in contrast to the usual broad shallow transverse gully profile of the clay soil group in humid regions. In permafrost areas different materials may yield the same cross-sectional profile. Gravel deposits ce— mented by ice may have vertical gully walls, or may have gullies'that are asymmetric in cross section if one side is preferentially exposed to the sun and th aws at a faster rate than the shadow side. The insulating materials overlying frozen deposits also affect the rate of thawing and hence the erosional characteristics of gullies. Interpreted carefully, however, the erc sional characteristics—gully cross section and gradient—may reveal useful information on texture and other engi- neering characteristics of the soil. Photographic tone On black-and—white photographs the significance of color must be interpreted in terms of relative gray photographic tones. Belcher stated (1948, p. 486) that 36 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING “Light color tones (grays) are usually associated with well-drained soils, while clays, principally because of their water—retention capacity, appear dar .” Frost (1946, p. 122), in describing the mottled soils in Indi- ana, stated that mottling nearly always indicates gravel. The mottling is due to “dark areas” which are “* * * small clay-like pockets that have been formed by the normal weathering processes of the gravel—sized material” (Frost, 1946, p. 122). These pockets are depressions that “* * * act as small infiltration basins which result in a higher moisture content and a darker color pattern” (Frost, 1946, p. 122). This mottled soil pattern contrasts with that of the drift plains in other areas where light-toned patches of slightly higher and dryer silty soils adjoin lower areas of wet plastic silty clays. Somewhat similar mottled patterns have been described by Gwynne (1942, p. 202—205) from the drift plain in Iowa (see fig. 107). But light photographic tones also may result from lack of moisture, owing to lack of precipitation as in many arid regions, rather than because of good drainage, and thus tone may be of little significance in evaluating soil texture in some areas. Or light tones may be the result of topographic position, and it is probable for example that a “* * * sand dune on a sand plain in a humid region will photograph light in color against a dark back- ground” (Purdue University, 1953, p. 26) not because of differences in soil textures but because of topo- graphic position. Climatic conditions also influence the colors of soils, especially the residual soils, and dark color rather than light color may be associated with well-drained materials; as certain red soils developed from limestone (Jenkins, Belcher, Greeg, and Woods, 1946, p. 84). Yet, provided climatic conditions are considered and provided relative photographic tone is borne in mind, the generalization that light photo— graphic tones suggest good drainage and dark tones poor drainage may be useful in the overall interpreta- tion of soils with regard to moisture content. Where tone is due to moisture, infrared photography will re— sult in strong tonal contrasts between soils of different moisture content. Color Color has been cited as a principal element of the soil pattern that may yield information on the texture and drainage character of the soil profile, but to date very little interpretive work has been done with color photography. Belcher (1945, p. 138) briefly mentioned experimental color photography taken in different parts of the United States and stated that “The value of color * * * lies in the added detail that it furnishes with respect to these two elements of the soil pattern” (that is, soil color and details of vegetative cover). It would seem that color photography would reveal pedo— logic information significant in engineering evaluation of soils, especially the residual soils, which may be more difficult to interpret in general than transported soils, particularly from black-and-white photographs. Vegetation Vegetation as an element of the soil pattern has been considered one of the most difficult to interpret. It is known, for example, that certain trees, such as aspen, are tolerant of many different soils and soil conditions, but some types of vegetation are strongly influenced by soil type. Furthermore, vegetation may be influenced significantly by climatic factors. Where permafrost is close enough to the surface to affect tree and shrub roots the kind of vegetation that will grow may be restricted (see p. 37). Species identification may be difficult in any area unless large-scale photographs are available or unless pure stands are present. Never- theless, vegetation may reveal significant information in engineering studies provided valid interpretations are made with regard to how vegetation is associated with soil textures, soil—moisture content, and topogra— phy; thus a knowledge of plant ecology is necessary if maximum information on soil conditions is to be ob- tained from aerial photographs. Willows, for example, indicate wet ground, poplar indicates dry ground, and jack pine implies sand and gravel beds (Belcher, 1945, p. 139). However, where abrupt changes in soil con— ditions exist, it is likely that vegetation changes will also occur (figs. 56 and 72), and areas needing further ground surveys may be easily delineated by inspection of aerial photographs without any basic knowledge of plant ecology. For example, stabilized sand dunes in interior Alaska can be easily delineated because they are marked by stands of dark-toned spruce (see fig. 56), in contrast to lightotoned brush and grass of the This relation may be found where the topographically higher, well-drained sand of the dunes contrasts with frozen silty deposits of the inter- dune areas. Where phreatophytes-plants and trees that obtain water from the water table—can be delineated (see p. 40), aerial photographs may provide significant engineering data because phreatophytes may choke overflow channels and thus increase the flood potential of some areas. Information pertaining to the down- stream de-silting effect of phreatophytes with regard to dams and reservoirs may also be obtained from aerial photographs of some areas (see Robinson, 1958, p. 28—29). However, little interpretative work in this field has been done. interdune areas. INTERPRETATION OF AERIAL PHOTOGRAPHS IN ENGINEERING GEOLOGY 37 Land use Land use may also reveal information on the soil conditions of an area. Belcher (1945, p. 142) reported that “Orchards thrive in well-drained locations and therefore, when observed on level ground, good sub- drainage is implied.” Shallow drainage ditches in areas of little relief commonly signify plastic, poorly drained soils. Schultz and Cleaves (1955, p. 378) stated that in eastern France and western Germany “* * * rugged topography and associated sandy soils developed on sandstones are generally left in forest; the comparatively level topography and associated clayey soils developed on shales and limestones are cultivated * * *.” On the flood plain of the Missis- sippi River the silty sand of the natural levees is culti- vated primarily because of its topographic position above the lower backswamps. In parts of the Valley and Ridge province of Eastern United States the lime- stone areas commonly are farmed whereas shale areas may be left in forest. PERMAFROST Permafrost has a significant effect on engineering characteristics of soils of the far northern latitudes and has received considerable attention in recent years, particularly from the standpoint of photointerpreta- tion (Cabot, 1947; Woods, Hittle, and Frost, 1948; Benninghoff, 1950; Frost and Mintzer, 1950; Frost, 1951; Sager, 1951; Black, 1952; Hopkins, Karlstrom, and others, 1955). Considerable disagreement exists, however, on the significance of photointerpreted data in the study of permafrost areas (see Black, 1952, p. 126—127, 129), although it is generally agreed that aerial photographs can be particularly useful in such studies. The following discussion of ground conditions in permafrost areas is taken from the work of Hopkins and Karlstrom (Hopkins, Karlstrom, and others, 1955), except as noted by specific references. Muller (1947, p. 219) defined permafrost as “a thick— ness of soil or other surficial deposit or even bedrock, at a variable depth beneath the surface of the earth in which a temperature below freezing has existed con- tinuously for a long time (from two to tens of thou- sands of years) .” Muller (1947, p. 30) also stated that “From the standpoint of engineering, the content of ice in frozen ground is of paramount importance * * * [because] ‘* * * thawing of this ice produces excessive wetting and undesired plasticity of the ground and renders it unstable and susceptible to settling, caving, and even flowing.” Building foundations, highways, railbeds, and airport runways may be considerably damaged unless the presence of permafrost is taken into consideration and measures taken to eliminate or minimize the effect of the thawing that results from the disturbance of the natural thermal regimen. Where bedrock or thick frozen gravel deposits that contain local drainage channels are the primary foundation materials, permafrost is not normally detrimental to construction. Although aerial photographs are useful in the inter- pretation of areas that may be underlain by perma— frost, criteria used for interpreting soil conditions of nonfrozen areas apply only to a limited extent to permafrost zones. Gully characteristics of an imper- meable gravel deposit cemented by ice, for example, will not necessarily be the same as the characteristics of gullies in impermeable nonfrozen soils. Because of the altered permeability and porosity of soils owing to permafrost, erosional characteristics, as applied to the interpretation of nonpermafrost areas, may have little significance in the perenially frozen regions. Other criteria, such as the common landforms, although sug- gestive of type of material present, are of limited use in determining the presence or absence of perm tfrost. Few diagnostic indicators of permafrost appear to exist. More often than not the criteria are suggestive rather than indicative; they include distribution and type of vegetation, polygonal relief patterns, pingos, and features resulting primarily from thawing. Distribution and type of vegetation Distribution patterns of trees and shrubs commonly supplement topographic expression in recognizing landforms; these patterns may contribute indire tly to the interpretation of probable permafrost conditions based upon inferences concerning the age of the land surface, climate, and character of the underlying ma— terial. In addition, the type of vegetation may be useful in delineating areas where permafrost is llikely to be present. Hopkins and Karlstrom (1955) re- ported that recognition of shallow—rooted species on aerial photographs aids in delineating the areas lwhere permafrost is most likely to occur at shallow depth, and that recognition of deep—rooted species (helps delineate areas least favorable for the formatibn or preservation of permafrost. Thus, black spruc may suggest areas underlain by permafrost at 5 allow depth, whereas tall willow shrubs and isolated pure stands of balsam popular on river flood plains gen- erally indicate unfrozen ground. The distributio‘ pat- terns of trees and shrubs, however, are most significant only when evaluated together with other information, such as type of landform, concerning permafrost dis- tribution in the region under study. Polygonal reliet patterns Polygonal relief patterns commonly can be iienti— fied on aerial photographs, but as they may be p1 esent 38 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING in both permafrost and nonpermafrost areas this cri- terion is not, by itself, always diagnostic in evaluating permafrost conditions; however, in conjunction with other features it may be very significant. Polygonal patterns may be divided into two broad groups: those formed by frost-stirring and those formed by con— traction. The formation of frost-stirred polygons is favored by the presence of permafrost at shallow depth, but these polygons are not necessarily an in— fallible indicator of shallow permafrost as they are also known to occur where permafrost is absent or is at a considerable depth. Contractional polygons are subdivided into low- center, high—center, and frost-crack polygons. Low- center polygons, one of the most dependable indicators of permafrost, are commonly found in poorly drained depressions. The presence of pools of water in the centers of some polygons is a diagnostic recognition feature, and it is common to find low-center polygons expressed on aerial photographs as dark-toned centers surrounded by light-toned marginal ridges (figs. 109— 112). High-center polygons occur in slightly better drained areas than those in which low—center polygons are found; generally, the centers are relatively level, although they may be domed where frost is extremely active. In contrast to low—center polygons, drainage around high-center polygons is commonly concentrated in marginal trenches, and pools of water are present at trench intersections. Because of the high centers, these polygons are commonly expressed on aerial pho- tographs as light-toned centers surrounded by dark- toned margins (figs. 40 and 109—112). Frost-crack polygons may be diflicult to distinguish on aerial pho- tographs, as they are similar to high-center polygons; they may be present in some areas where permafrost is absent. Other relief patterns in conjunction with vegetation distribution result in striped slopes that are commonly present in permafrost areas (fig. 40). Pingos Pingos are rounded or elliptical steep—sided hills formed by the “downward freezing of a body or lens of water or of semi-fluid mud” (Muller, 1947, p. 59). They may be as much as 300 feet high. Fissures may develop along the axis of a pingo or radiate from the crest. Pingos usually are found in poorly drained areas and normally are readily distinguished on aerial photographs by their distinctive local landform (figs. 110 and 111) ; they are almost always a reliable indi— cator of permafrost, although locally they may be con- fused with erosional knolls or hills. Features resulting from thawing Two features associated with thawing are suggestive of the presence of permafrost. These are thaw lakes (sometimes called thermokarst lakes) and beaded drainage. Thaw lakes occur in frozen fine-grained sedimentary deposits that generally contain clear lenses and masses of ice whose volume is greatly in excess of the porosity of the unfrozen material. As this ice melts the ground subsides and forms a basin in which a thaw lake may accumulate (figs. 110 and 111). When caving is progressing, banks are ragged and steep or overhanging. Tilted trees known as “drunken forests,” along the margins of such lakes are indicative of active caving. In treeless areas active caving may be indicated by tension cracks parallel to the banks. Other indications of thawing, such as serrated margins of lakes, also may be present (figs. 109—111). The usefulness of thaw lakes as permafrost indicators, however, is limited by the difficulty of distinguishing on aerial photographs between active thaw lakes, relict thaw lakes, and similar-appearing lakes that are in no way related to permafrost. Beaded drainage occurs particularly in perennially frozen peat and silt containing ice wedges; it is char- acterized by small pools connected by short water- courses that may be sharply incised (figs. 40 and 112). The pool banks commonly are steep as a result of thawing and caving of frozen materials. Absence of permafrost In areas of discontinuous permafrost it is not only desirable for the engineer to know where permafrost is present, but it may be equally important to deter- mine where permafrost is absent. Certain hydrologic phenomena and elements of the soil pattern commonly indicate or suggest the absence of permafrost. For example, subterranean drainage in unfrozen zones within permafrost may be detected on aerial photo- graphs of some areas. The recognition criteria are in part similar to those applied in evaluating soil condi- tions in nonpermafrost areas of the world. That is, streams may disappear into the bottoms of closed de- pressions or by percolating into a gravel—covered sur— face. In addition, the presence of dry depressions in permafrost areas may indicate locally well-drained areas, inasmuch as there is generally a source of water during the summer months. Such areas contrast with other areas of the earth’s surface Where dry depres— sions may be due to absence of precipitation or other immediate source of water. Locally unfrozen zones within permafrost areas may also be suggested by springs, which are commonly indicated by flo d-plain icings, shown as flat, white surfaces on som aerial INTERPRETATION OF AERIAL PHOTOGRAPHS IN ENGINEERING GEOLOGY 39 l photographs, or indicated by more luxuriant vegeta— tion along a pronounced lineation or within a dark- toned area on low hill slopes. These unfrozen zones may be favorable as a source of ground water. In addition, because permafrost generally causes soils to be impermeable, drainage characteristics indicative of permeable materials (see p. 34) may be interpreted to mean lack of permafrost, at least in the immedi- ately underlying materials. Certain tree species may also suggest absence of permafrost (see p. 37). EROSION, TRANSPORTATION, AND DEPOSITION LANDSLIDES Natural movement or potential movement of ma— terials is a significant consideration in many construc- tion engineering problems. Foremost in this regard is the movement of soils and rock materials by land- slides. Ritchie (1958, p. 67) stated that “All landslide investigations must start with recognition of a dis— tressed condition in the natural or artificial slope * * *.” Distressed conditions are commonly interpretable from aerial photographs. “The evidence for distressed con- ditions that may be present, or that may be induced, lies chiefly in evidence of movements, minor or major, that have already taken place or of geologic, soil, and hydrologic conditions that are likely to cause move— ment in the future” (Ritchie, 1958, p. 67). Geologic, soil, and hydrologic conditions signifying potential landslide areas include the presence of unfavorable geologic structures and rock type, prevalence of fine- grained materials, and an abundance of water or con- ditions that would permit access of water to fine— grained materials. Because remedial measures for landslides or preven- tion of landslides are generally not only difiicult but costly, avoidance of landslide areas is important in highway route layout. Existing landslides are com- monly identified on the basis of landform (figs. 46 and 113). Liang and Belcher (1958, p. 70) stated that some landforms are more susceptible to landsliding than others, and thus identifying the landform is highly important. “Potential slides of the rockfall and soilfall type can commonly be foreseen simply by recognizing geologic conditions that are likely to pro- duce overhanging or oversteepened cliffs” (Ritchie, 1958, p. 51). Basalt cappings underlain by easily eroded shale (figs. 53, 87, and 113) illustrate a condi— tion where rockfalls may occur as a result of failure and slump in the underlying materials. Movement of unconsolidated materials by fiowage is likely to produce the most damaging landslides. Ex- isting flows and slides (figs. 46 and 113) commonly have a “hummocky” surface. Landslide scars, where soils and vegetation have been stripped off tlie bed— rock, may be easily recognized on aerial photographs by the light photographic tone of the bedrock in con- trast to darker tones of surrounding areas. A eas of potential flow of unconsolidated materials may1be re- vealed by tension cracks or crevices, or suggested by the presence of seepage zones or springs, shoiwn by dark tones of dense or luxuriant vegetation or bi dark tones of the soil. if, Ritchie (1958, p. 50) warned, however, that although aerial photographs are of significant help in d velop— ing the setting for detailed studies, they seldo con— tain the detail needed by the engineer to car1 out preventative or remedial measures. The prime (appli- cation of aerial photographs in landslide studies ap- pears to be in interpreting ground conditions toideter- mine areas that should be avoided in highway} route layout and similar problems. i i BEACH EROSION i In a shoreline study in California, Munk and‘Tray- lor (1947 ) showed that variation in wave heig t and refraction of waves along the shore was controlfled in part by sea-floor topography. Where refraction : auses a convergence of waves the rate of shoreline e osion may increase. Krumbein stated (1950, p. 203 that “Refraction diagrams are becoming an essentia part of any study concerned with shore processe , and graphic methods have been developed for preparing them from hydrographic charts or aerial photographs.” Photographs taken periodically of shoreline (areas would also provide information, both qualitatifie and quantitative, in studies of erosion and deposition, which might thus be useful in planning remedial action against the eroding currents. In a study of shoreline erosion by the U.S. Army Corps of Engineers ((U.S. Beach Erosion Board 1946, p. 13) aerial photographs taken in 1941 were compared with land survebrs of 1836 to obtain rates of erosion along the shore of; Lake Michigan. The rate of erosion plays a significant part in planning corrective measures for eroding culi‘rents. BEDROCK GEOLOGIC STRUCTURE AND TYPE OF ROCK ‘ Because of the influence that faults may have on construction design and costs, it is important} that faults be located and studied in the beginning stages of an engineering project. Aerial photographsi may provide significant information in this regard aslsome faults are recognized from subtle expressions on photo- graphs but are identified only with difficulty oh the ground. The usefulness of photographs in mappjfing a proposed damsite in southeastern Alaska is shown) in a 40 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING study of the Swan Lake area (fig. 114) where faults stand out in aerial view as conspicuous lineations, which are primarily reflections of rectilinear depres- sions on the ground. In a damsite study in Malaya, Alexander and Proctor (1955) used large—scale aerial photographs taken 750 feet above the ground. These photographs showed clearly the attitude and distri— bution of faults and joints in granitic country rock. In connection with the San J acinto tunnel project in California, Henderson (1939) noted that aerial pho- tographs revealed indications of unsuspected faults and corroborative evidence of those previously sus- pected. Fault locations underground were found to be close to that predicted from study of photographs. In addition to showing faults, aerial photographs commonly shown dipping beds that are important in some foundation studies for pier and abutment loca- tion. Or dipping beds may indicate potential rock- slide conditions where the dip is in the same direction as the hill slope. Lithologic characteristics of bedrock materials are also significant in many engineering problems, as in foundation work, or in locating road metal sources. Criteria useful for interpreting the general lithologic character of rock types from aerial photographs are discussed on pages 16—19. INTERPRETATION OF AERIAL PHOTOGRAPHS IN HYDROLOGIC STUDIES LOCATING POTENTIAL GROUND-WATER SOURCES Little has been written on the interpretation and uses of aerial photographs specifically in hydrologic studies, although photogeologic techniques offer both practical and research applications in this field. Prac- tical application of aerial photographs in hydrologic study are at present confined largely to assisting the geologic mapping in ground-water investigations, par— ticularly in areas covered by surficial materials. As an aid in ground-water mapping, elements of the soil pattern are evaluated in terms of ground conditions, just as in many engineering geology studies (see p. 34—37). Thus, for example, coarse—textured drainage or absence of drainage may signify highly permeable materials (figs. 52, 66, 88, 104, and 1058; and p. 34—35) or the landform (figs. 52 and 104) may suggest the kinds of materials that compose it, which permits an evaluation of permeability and porosity and potential as a water reservoir. In addition, aerial photographs may reveal information directly suggesting the pres— ence of water, such as a preferential distribution of vegetation at the margins of a gravel cap (see figs. 104 and 108). The type of vegetation in turn may permit inferences with regard to the general quality of water. For example, salt cedar is tolerant of rela- tively high content of salt in water, whereas cotton— wood trees are intolerant. Howe (1958) described briefly the use of aerial photographs with specific ref- erence to locating possible water-bearing formations. WATER-LOSS STUDIES In a study of water loss caused by phreatophytic plants and trees, which take their water from the water table, Turner and Skibitzke (1952) used aerial mosaics and contact prints for delineating areas of different densities of phreatophytes. The areas were then measured by planimeter and used in conjunction with vertical foliage density, determined by field methods, to arrive at volume density to which water loss by transpiration for specific species could be equated. Data pertaining to tree species and heights were determined by ground methods. Turner and Skibitzke (1952, p. 67) stated that “The comparison between results obtained by air and by ground map- ping led to the conclusion that the former offers a much faster and more accurate method, particularly in areas of dense growth.” Further useiof aerial pho- tographs in collecting basic data for studies of water loss by phreatophytes could probably be made. For example, it may be possible to determine tree or plant species, especially when large—scale black-and-white photographs are available or where color photographs have been taken. In addition, for many areas where the ground surface can be observed through the trees, photogrammetric measurements may provide rapidly those height measurements needed in water—loss studies. For areal measurements of differing vegeta— tion types or densities a simple dot-templet method may be useful (Wilson, 1949). Like many other studies, however, where vegetation and ground fea- tures must be observed, measured, and plotted, maxi- mum use of aerial photographs will result from com- bined use of field and photogeologic methods. OTHER APPLICATIONS Aerial photographs as a tool in hydrologic studies provide a medium for obtaining basic data, such as cross—section profiles, linear, areal, slope, and volume measurements. Some of the suggested uses of photo- grammetric techniques in hydrologic studies are: (a) for definition of channel size and shape of cross sec- tion to be used in studies of the effect of channel geometry in flood characteristics of streams; (b) for determining channel storage capacity used in flood— frequency correlations; (c) for definition of the three- dimensional sinuosity of natural channels to be used in studies of the rate of energy loss; ((1) for definition of the geometry of bed roughness of alluvial channel KINDS OF INSTRUMENTS models (Thompson, 1958); (e) for measuring topo- graphic characteristics of drainage basins such as stream slopes and drainage density; and (f) for de— termining areas of lakes and swamps in water-storage studies. INSTRUMENTATION Instruments in photogeologic study serve three ob- jectives, namely, interpretation, measurement, and plotting of data. The instruments used thus may be photogrammetric or nonphotogrammetric—photogram— metry is the art of making reliable measurements from photographs. Usually different instruments are used for attaining each of the three objectives, as, for example, in using a stereoscope, parallax bar, and ra- dial planimetric plotter, respectively. But all three op- erations may be combined in a single piece of equip- ment, such as the Kelsh plotter (fig. 18). Whether interpretation is done with a simple stereoscope and paper prints or with a precision stereoplotting instru- ment using glass-plate diapositives, the criteria of rec- ognition, described in the first part of this paper, re- main the same except insofar as scale and resolution of photographic details are involved. Both measure— ment and plotting are here considered as mechanical aids that provide quantitative information to be fur- ther used in geologic interpretation of an area, and in- struments are discussed primarily with this objective in mind. Measurement involves (a) the determination from aerial photographs of vertical and horizontal distances, which in turn are used to compute stratigraphic thick- nesses, angles of dip, and other quantitative data of geologic use; (b) the direct determination of inclined distances, commonly the stratigraphic thickness of a formation; (0) the direct determination of angles of slopes or beds; and (d) other quantitative determina- tions of possible significance in geologic interpretation, such as light-transmission measurements. Plotting primarily involves the orthographic posi— tioning of geologic data from aerial photographs to base maps or compilation sheets. It also involves direct plotting of geologic data in cross-sectional Views, and may include positioning data such as geologic contacts, which are not directly recognizable in the stereoscopic model, by use of the floating dot or spatial reference mark that is found in many photogrammetric instruments. KINDS OF INSTRUMENTS Many different photogrammetric instruments are available for making measurements and plotting geo- logic data to base sheets or cross-sectional profiles (see Fischer, 1955; Ray, 1956; Fillmore, 1957; and Hemp- (41 hill, 1958a). Certain instruments permit only the measurement of altitude differences, others perm't only the orthographic positioning of geologic data, but many instruments are used for both measuri g and plotting. Most measuring and plotting devices are designed to accommodate standard 9- by 9-inch j hoto— graphs, enlarged photographs can be used wit only a few instruments, such as the overhead projector, in transferring data to a base sheet. MEASURING DEVICES FOR USE WITH PAPER PRINTS One of the most commonly used and Widely avail— able types of instrument for determining altitudes from paper prints of aerial photographs is the stereom- eter or parallax bar. Several varieties of stereom- eters are available for use with either the lens or mirror-type stereoscope (see fig. 10); they consist of two small plates, usually of glass or plastic, that have inscribed dots, or other targets, that can be centered over conjugate image points in a stereoscopic pair of photographs. The plates are attached to a supporting bar along which they may be separated horizontally. The supporting bar generally has graduated readings in millimeters or inches and a slow-motion adjustment drum that permits readings of hundredths of milli- meters or thousandths of inches. All stereometers are based on the “floating-dot” principle, where two target dots, one seen with each eye, are fused stereoscopically into a single dot that appears to float in space within the stereoscopic model. The apparent height of the single fused dot, used as a reference mark in deter- mining altitude differences, is related to the horizontal separation of the individual dots being viewed. Thus by measuring the horizontal separation between indi— vidual dots when the fused dot in the stereoscopic model is placed at the top of an object (such as a hill or cliff) whose height is to be determined, ani sub- tracting from it the measurement of the horizontal separation between individual dots when the fused dot is placed at the bottom of that object, a measure of the height, called differential parallax, is obtained. Dif- ferential parallax is converted to feet by simple mathe— matical calculations (see p. 53). Stereometers are thus merely devices that permit reliable measurement of differences of horizontal distances between two or more pairs of conjugate image points as seen on two photographs that form a stereoscopic pair. Parallax ladders also may be used for determining differences in altitudes from aerial photographs. Like the stereometer, the parallax ladder is based n the floating-dot or floating-line principle. The instrument may cOnsist of two diverging rows of dots on plastic or glass arms; thus, a series of pairs of dots exist with 42 different horizontal separations, and this in turn re- sults in several floating dots all at different altitudes within the stereoscopic model (see fig. 12). Or the instrument may consist of two diverging lines with intercepts of specific horizontal separations ticked ofl’; thus a single plunging line would be seen in stereo- scopic View. The instrument, oriented at right angles to the flight line, is used by sliding it over the stereo- scopic model until a pair of dots or line intercepts— seen stereoscopically as a single dot or cross in space— appears to fall on the base of the object whose height is to be determined. A reading of the horizontal separation of the two dots or intercepts is then made. The parallax ladder is moved until a different pair of dots or interceptsflalso seen stereoscopically as a single dot or cross in space—appears to fall on the top of the object whose height is to be determined. Again a reading of the horizontal separation of dots or inter- cepts is made. The difference in readings obtained is then used in a simple formula to determine the dif- ference in altitude of the two points (see p. 53). A direct-reading parallax ladder that eliminates the need for most of the mathematical computation has also been devised. The differences in horizontal sepa- ration of pairs of dots has been calculated by the manufacturer in terms of heights in feet for different flying heights and photobases, and this permits a di— rect reading of relative altitudes within the stereo- scopic model. Also for use with paper prints is a stereo slope meter, designed primarily for determining degrees or percentage of slope. The instrument consists of two transparent disks each scribed with eccentric circles of specific diameters (fig. 11). These disks are mounted in a frame so as to be movable horizontally in a fash- ion similar to the dots of the stereometer. In stereo- scopic view the two sets of eccentric circles appear as a cone that is divided into several zones. By proper horizontal spacing of the two disks, the resulting cone seen stereoscopically can be raised or lowered so that some two circles will rest on the slope or grade to be determined. By appropriate simple calculations the slope can be determined. Center dots present on each of the transparent disks also permit relative altitudes to be determined just as with a stereometer. PLOTT‘ING DEVICES FOR USE WITH PAPER PRINTS Instruments have been designed specifically for plotting information from paper prints to base maps or control sheets. An instrument widely employed in the past is the sketchmaster (fig. 13), which uses the camera-lucida principle in transferring data from the photographs to the base sheet. The sketchmaster AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING allows the operator to view a single photograph image superposed on the base map. Adjustments for scale changes in the perspective photograph permit coinci- dence or near coincidence of photograph control points and base-map control points. Geologic detail is sketched directly on the base map. This instrument can be adjusted to remove small amounts of tilt in— herent in some vertical photographs, but like any direct—reflecting projector, large amounts of radial dis- placement due to relief cannot be effectively removed. The radial planimetric plotter has been devised es- pecially for transferring photographic detail to a base map or control sheet. Unlike the sketchmaker, this instrument is designed to eliminate displacement due to relief. The plotter consists of a mirror stereoscope mounted above two photograph tables (fig. 14). A transparent plastic arm with a centrally scribed line extends from and pivots around the center of each table. These arms are linked to a pantograph attach- ment. Because the plastic arms radiate from different centers they cross each other in stereoscopic view to form the so-called plotting cross. In operating the radial planimetric plotter a pair of vertical photographs is oriented on the photograph tables for proper stereoscopic viewing, and control points on the photographs are oriented to control points on the base manuscript by means of the radial arms and pantograph. Movement of the pantograph attachment moves the plotting cross over the stereo- scopic model and permits tracing of photograph detail on the base manuscript. Inasmuch as the radial arms that intersect. a terrain feature on each photograph represent azimuth lines from known points on the base manuscript, the intersection of these two arms will represent the true map position of that feature, just as will the intersection of two azimuth lines shot from different instrument stations in the field. Thus the relief displacement of a feature on a photograph is effectively removed; this is one of the chief advantages in using the radial planimetric plotter. However, the radial lines do not intersect along the principal line—or flight-line direction—between photographs, and the photograph tables must be shifted to their al- ternate centers before the central area of stereoscopic model can be delineated. Because the photograph tables are mounted in a horizontal position, the instru— ment does not permit removal of tilt that may be pres- ent in the photographs, but tilt is usually small in pres- ent-day photography and generally does not cause significant errors in horizontal positioning of geologic data. The multiscope is a combination of mirror stereo- scope and camera lucida that has received limited use KINDS OF INSTRUMENTS FIGURE 10.—Stere01neters. FIGURE 11.~Stereo slope meter. Fm [IRE 12.-Parallax ladder. MEASURING INSTRUMENTS FOR USE WITH PAPER PRINTS 553966 0—60—3 43 44 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING FIGURE 13.—~Sketchmaster. FIGURE 14.—Radial planimetric plotter. PLOTTING INSTRUMENTS FOR USE WITH PAPER PRINTS FIGURE 15.~Stere0t0pe. COMBINED MEASURING AND PLOTTIN G INSTRUMENT FOR USE WITH PAPER PRINTS KINDS OF INSTRUMENTS in plotting geologic data from photogIaphs to base maps 01 control sheets. Photographs are mounted on movable photograph plates that permit small adjust- ments for tilt. The viewing assembly is constructed to allow the insertion of one or two half-silvered mir- rors in the eyepiece, so that in operation either the image of a single photograph or the stereoscopic model may be superposed optically on the map or control base. Although the stereoscopic model may be seen at all times, use of only one half-silvered mirror permits the eye to see only the image of one photograph super- posed on the map base; the resulting plot is similar to that from the sketchmaster—or reflecting projector —no errors due to relief displacement are removed. If, however, two half-silvered mirrors are used, appro- priate manipulations permit true orthographic plotting of detail from the stereoscopic model. Scale adjust- ments between photographs and map base are made by interchanging special lenses in the viewing as- sembly together with changing manually the distance of the viewing assembly above the map base. The instrument is said to permit measurement of altitudes when coupled rotary prisms are inserted under the half-silvered mirrors, or when a device similar to the multiplex tracing table is placed in the field of stereo- scopic view (Spurr and Brown, 1945, p. 177). MEASURING AND PLOTTING DEVICES FOR USE WITH PAlPER PRINTS Some instruments have been designed both for meas- uring altitudes and for plotting data from paper prints of aerial photographs. These instruments are com- monly termed “paper-print plotters” and include the KEK plotter, the Mahan plotter, and the stereotope. The KEK plotter consists of a stereoscope, two photograph tables. floating-dot assembly, and drawing attachment. The plotting cross of the radial plani— metric plotter is replaced in the KEK plotter by the fused dot floating in space. By raising or lowering the photograph plates the fused dot» is positioned on the ground in the stereoscopic model. Vertical motion of the photograph plates is linked to a drum scale on which relative altitudes can be read directly in feet. Movement of the pantograph drawing attachment allows geologic detail to be sketched directly on the map base. but during this sketching the fused dot must be held 011 the ground in the stereoscopic model by simultaneous vertical movement of the photograph plates. The photograph plates may be tilted to make an approximate correction for tilt that may be inher- ent in the photography. Because the floating-dot as- sembly lies above and is physically separated from the photograph tables. small amounts of extraneous paral- 45 lax and significant amounts of horizontal shift in the map position of a point may result by moving one’s head in viewing the stereOscopic model. The Mahan plottel is generally similar in principle to the KEK plotter but differs slightly in operation. The floatng dot is positioned on the ground in the stereoscopic model by vertical motion of the disks on which the dots are scribed, whereas in the KEK plotter the position of the disks containing the scribed dots is fixed and the photograph plates are moved vertically in order to position the floating dot at a particular level in the stereoscopic model. The stereoscopejof the Mahan plotter is adjustable, which permits very nearly the recovery of the perspective from photography ranging in focal length from about 8.25 to 12 inches. As with the KEK plotter, a shift of the viewer’s head when viewing the model may cause horizontal posi— tioning errors and the presence of small amounts of extraneous parallax. The stereoscopic model of the KEK and Mahan plotters may be effectively leveled, in absence of vertical control, by visual reference to certain physiographic features of the terrain see p. 55). ‘ A somewhat different paper—print plotter 1s the stereotope. The instrument consists of a stereoscope with binoculars of X 4 magnification mounted over a photoholding assembly that contains a parallax bar and attachments for pantograph hookup, as well as a mechanism, when vertical control is available, for effectively leveling tilted models, for correcting errors of horizontal position, and for orthographic plotting of geologic data (fig. 15).~ The stereotope is an elabo— rate instrument that corrects for tilt by mechanical linkages within the photoholding assembly. No physi- cal tilting of the photoholders takes place and thus the instrument differs significantly from the KEK and Mahan plotters. In addition, the targets of the stereom— eter attachment for altitude determinations are in contact with the paper prints, and no extraneous parallax can be introduced by movement of the oper- ator s viewing position. In the absence of Wertical contml, which permits tilt correction through mechani- cal linkages. the stereotope as a measuring device must be used as a simple parallax bar. MEASURING AND PLOT'I‘ING DEVICES FOR USE WITH GLASS- PLATE DIAPOSITIVES The Kelsh plotter, multiplex, and ER—55 plotter, all of which require glass-plate diapositives, are used in the United States both to measure quantitative geo- logic data and to plot geologic detail to base maps or control sheets (figs. 16—18). These double-projection precision—plotting instruments are designed to accom- 46 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING ‘ n 3 .~ 1," IA. Plum 16 Mu Up 8‘ FIGURE ]7.——ER755 plotter. FIGURE 18.~Ko]sh plotter. COMBINED MEASURING AND PLOTTING INSTRUMENTS FOR USE WITH GLASS-PLATE DIAPOSITIVES KINDS OF INSTRUMENTS 47 modate photography of specific focal lengths, usually 6 or 8.25 inches. All utilize the anaglyph principle of projection of light of complementary colors, red and blue-green, through glass—plate diapositives to create the third dimension. The stereoscopic model is viewed through glasses of the same color as the filters used in the light-source projectors. Because double-projection, or anaglyphic-type, stereoplotting instruments simu— late in miniature'the spatial relations of the camera stations at the time the photographs were taken, all features of the terrain are optically re-created in the stereoscopic model in essentially true relation with respect to the measuring mark or floating dot. Where sufficient vertical control is available any tilt inherent in the photography can be removed so that accurate measurements of altitudes can be made and true ortho- graphic positioning of detail obtained. Without verti— cal control the stereoscopic model of many terrains commonly can be leveled within 10 of the horizontal datum by visual reference to certain physiographic characteristics of the terrain (see p. 55); under these conditions approximate orthographic positioning may be obtained, but significant errors in vertical measure- ments may result (see p. 69—7 5). The stereoscopic model is usually viewed on a small white-surfaced table called a platen, which may be raised and lowered so that an illuminated floating dot in its center is kept in contact with the surface of the ground, as seen in stereoscopic view. Vertical motion of the platen is transmitted to a scale reading in milli- meters of vertical measurement, or, on some instru~ ments, to a scale that converts readings directly to heights in meters or feet. Geologic features are traced orthographically 011 the base map by a pencil located directly beneath the illuminated dot. on the platen, or by a reduction pantograph attached to the tracing table. Interpretation, measuring, and plotting can be carried out in one continuous operation. The Kelsh plotter is designed to accommodate glass- plate diapositives the same scale as the original pho- tography. For many geologic problems film positives may be substituted for the more expensive glass-plate diapositives. The scale of the projected stereoscopic model is usually about 5 times that of the original photography. Only that part of the stereoscopic model appearing on the platen is illuminated; this results in a concentration of light and a brightly illuminated model but prevents viewing of the entire model at one time. Because the glass—plate diaposi— tives are the same scale as the original photography the resolution of the stereoscopic image is excellent. The multiplex uses glass- plate diapositives on which the (niginal 9- by 9- inch negative is reduced about 5 times. The small diapositive image is then enlarged approximately 12 times in projection of the jstereo- scopic model. The projected stereoscopic model is about 2.5 times the original photography scalej As a result of the large amount of reduction of the original photograph negative, some of the photographic} detail is lost and fine details of terrain, important in geologic interpretation, may not be visible. In nmltipl 3x pro- jection the entiIe model meat is illuminated, anti if the teIrain being viewed has only low or moderatel relief the stereoscopic model may be observed 1n its ef1ti1ety by substituting a large white surface for the platen. This overall view of the stereoscopic model is often of considerable use in geologic interpretation, because of the association of geologic features that can be seen at one time. The overall View is generally accom- plished by plojecting the stereoscopic model to the multiplex table slate; no plotting or measuring [an be done when the model 1s viewed on this surface. ‘ The Ell—55 plotter uses glass-plate diapositij‘es 011 which the original 9- by 9-inch negative is reduced about 2.8 times; this amount of reduction does not seriously affect image resolution. The projected stereo— scopic model is about 3.5 or 5 times the scale bf the original photographs, depending 011 the projector model. Ellipsoidal reflector-type projectors result in a brightly illuminated model in which terrain detail has a high degree of resolution. Like the multipleix, the ER—55 plotter permits viewing the entire steredscopic model at one time. In addition to accommodating vertical photography the ER—55 projectors are adapta- ble to twin low-oblique photography taken with the camera axis inclined 20° from the vertical position. Geologic study of stereoscopic models in double- projection plotters has numerous advantages over the study of paper prints. The ability to interpret, meas- ure, and plot in one continuous operation has already been cited. In addition, the geologist works with an enlarged stereoscopic model that ranges from labout 2.5 times to .) times the original photographyi scale, depending 011 the instiument used, thus small— scale photography commonly may be used A higher degree of 1111age 1esolution is maintained on glass diapositives than 011 paper prints, and results in more detail that can be obsened in the double- -projection stereo copic models except for the multiplex, 111 which somei reso— lution needed in qualitative and quantitative int Iple‘ tation is lost in the projected model. Becau e all features of the terrain are optically re-created in ssen- tially true relation, reliable 111easuren’1ents for str cture contouring, for drawing isopach lines, for commuting strikes and dips, and for determining fault displace- ments can be obtained for many areas with stereo- 48 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING scopic plotters; measurements can be made rapidly and hence economically (see Pillmore, 1957). OTHER INSTRUMENTS Other photogrammetric instruments such as the stereoplanigraph and autograph permit precise meas— urements from aerial photographs, but these heavy plotters are generally unavailable to the geologist. Beside permitting precision of measurement, these in- struments are designed to accommodate different sizes of diapositives and photography taken with different focal—length lenses. Certain modifications or additions to existing instru- ments have been made specifically for geologic pur- poses; and some new instruments, described below, have been designed expressly for geologic study. EXAGGERATED-PROFILE PLOTTER The exaggerated-profile plotter is a device attached to the tracing table of double-projection plotters, such as the Kelsh, multiplex, and ER—55 plotters, for draw- ing terrain profiles. As the tracing table is moved along the line of. profile and as the platen of the trac- ing table is moved vertically a pencil attachment records the profile on a vertical tracing board. Profiles exaggerated as much as 5 times may be plotted. An arm on the tracing table slides in a groove along the base of the tracing board so that a selected cross— section direction is maintained. Two models of the exaggerated-profile plotter have been developed. One is based on the lever and fulcrum principle (fig. 19) ; the other is based on the pantograph principle (fig. :20). The exaggerated-profile plotter was first devel- oped to aid in solving a specific geologic problem in correlating thin and closely spaced intraformational sandstone beds in rocks of Cretaceous and Jurassic age in Wyoming (Pillmore, C. L., oral communication, 1958). The plotter may be very useful also in quanti- tative geomorphic studies or in studies involving the qualitative correlation of shape characteristics of terrain. INTERVAL-MEASURING DEVICE A floating—dot instrument that permits the mainte- nance of a desired ve ical‘interval between two float- ing dots in a stereoscopic model from paper prints, as well as serving as a conventional stereometer, also has been developed in the course of photogeologic study (Hackman, R. J.. oral communication, 1959). Two floating dots, representing different altitudes in the stereoscopic model, may be useful for example in posi- tioning an obscured formation contact when other con- tacts are readily observed and formation thicknesses are known. The instrument consists of two transparent circular disks mounted on a frame with two worm screws, one of which permits separation of the disks in the a? direction, and the other of which permits both disks to be rotated equally but in opposite directions (see fig. 21). A series of dots, evenly spaced, are ar- ranged in a straight line so as to pass through the center of each disk. One dot coincides with the cen— ter of each disk and can be moved only in the an direc- tion, as in most stereometer-type instruments. The other dots may also be moved in the plane of the disk by rotating the disk, thus providing a parallax ladder as seen stereoscopically. By proper adjustment a pair of dots, one on each disk, can be separated by rotation of. the disks to provide a direct-reading parallax lad- der, or the dot separation may be referred to millime- ters of parallax and differences in altitude calculated in the usual manner (see p. 53) The center dots of each disk can be floated in the stereoscopic model at a desired point and the distance above or below this point measured merely by rotation of the disks. Scales graduated to record hundredths of millimeters of paral— lax permit readings of separation of the center dots of the disks as well as reading of total 90 parallax re— sulting from rotation or 90 motion of the disks. UNIVERSAL TRACING TABLE Some research in development of instruments car— ried out to date has centered around the modification of existing instruments. Important in this group is the universal tracing table (fig. 22), which is a multi— plex tracing table that has been modified in design by R. H. Morris and C. L. Pillmore of the Geologic Sur- vey. It is used for direct measurement of inclined distances, such as stratigraphic thickness of dipping beds, in stereoscopic models from double-projection plotters. The platen and underlying assembly can be tilted as much as 45°. A separate worm-screw drive on the tilted assembly distinct from the conventional worm-screw drive for vertical movement, permits movement of the platen in an inclined direction per— pendicular to the surface of the platen. A scale on this auxilliary worm-screw-drive assembly permits the reading of measured intervals in hundredths of milli- meters; conversion to feet is made in the usual manner for double—projection stereoscopic models (see p. 54—55). A series of holes drilled through the platen to the underlying light source provides a plane of floating dots in the stereoscopic model. MEASUREMENT PRINCIPLES OF VERTICAL. MEASUREMENT Vertical measurements of altitude differences pro— vide by far the greatest amount of quantitative in- formation in geologic interpretation from aerial pho- tographs. Differences of altitudes are generally deter- MEASUREMENT 49 FIGURE 19,—Lever and fulcrum type of exaggerated-profile FIGURE 20.#Pantograph type of exaggerated-profile plotter. plotter. FIGURE 21,—Interval-Ineasuring instrument. FIGURE 22.—Universa1 tracing table. INSTRUMENTS DESIGNED OR MODIFIED FOR GEOLOGIC USE 50 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING mined from overlapping aerial photographs, although they may under special circumstances be determined from a single photograph. When overlapping prints are used, the altitude difference between any two points is determined by measuring the horizontal linear parallax difference between the two points and relating it to appropriate geometry of the stereoscopic model. Altitude determinations from glass plates in double— projection plotters require the measurement of actual vertical distances within the stereoscopic model. In the uncommon circumstance when a single photograph may be used, altitudes are determined by measuring relief displacement and relating it to appropriate geometry of the aerial photograph. DETERMINATION OF ALTITUDE DIFFERENCES BY THE PARALLAX METHOD \Vhen an object is Viewed or photographed from two different positions, as on two overlapping vertical aerial photographs, an apparent shift in the position of that object takes place, which is known as paral- lactic displacement. On overlapping aerial photo- graphs this is a measurable linear distance that is directly related to the height of the object. In photo— grammetric terms parallax of an image point appear- ing on two overlapping photographs is called absolute stereoscopic parallax and is represented by the alge— braic difference, parallel to the flight line, of the dis- tances of the conjugate image points from their re- spective principal points (see fig. 23). The difference in absolute stereoscopic parallaxes between two differ- erent image points is a measure of the distance one point is above the other. On overlapping paper prints the parallax diflerence may be measured reliably by using a stereometer-type instrument, based on the floating-dot principle; parallax difference may also be measured with a ruler, although this procedure is rarely used because of inaccuracy of measurement. In double-projection stereoplotting instruments (Kelsh, multiplex, and PER—55 plotters) an actual ver- tical measurement of altitude difference is made within l Principal poini,!eft photograph Principal point, right photograph Flight line X FIGURE Eli-Diagram showing relation between absolute stereoscopic parallaxcs and horizontal distances actually measured with stereometer-type instruments in determining differences in altitude from paper prints. Let images P1’—P2’ and P1’—P2’ represent the photographic expression of a pole on the left and right photographs of a stereoscopic pair. The absolute stereoscopic parallax of the base of the pole is z— (—x’)=(x+x’). The absolute stereo- scopic parallax of the top of the pole is y— (—y’):(y+y’). (Distances measured to the right of the principal point are positive, to the left, negative). The parallax difference between the top and bottom of the pole then is (yi— 71’)— (x—l—x’). From the figure: y+z/’+B:x+x’+/l y+y’—x—x’=A—B (y+y’l — (;r+z’) :A — B. Thus the dlffcrence in absolute stereoscopic parallaxes is equal to A—B. The distances A and B are the actual distances measured with stercometer-type instruments, gen- erally in hundredths of millimeters, in determining altitude differences from paper prints. MEASUREMENT 51 the stereoscopic model; the linear parallax recorded on the photograph is translated into a vertical dis— tance as a result of angular parallax in the viewing arrangement. Figures 23 and 24 show diagrammati- cally the distances that are actually measured when stereometer—type instruments and double-projection instruments are used respectively in altitude deter- minations. Light source USE OF STEREOLIET‘ER-TYPE INSTRUIWENTS When paper prints of aerial photographs are used in conjunction with a stereometer-type instrument and a stereoscope, two linear horizontal distances must be measured to obtain the parallax difference between two different image points. These distances are simply A and B as shown on figure 23. They can be me isured with a finely graduated ruler but more comm nly a Light source Diaposiiive Light rays\ D FIGURE 24,—Diagram showing relation between absolute stereoscopic parallaxes and vertical distance measured with double- projection type instruments in determining differences in altitude from glass plates The absolute stereoscopic parallaxes of the top and bot: tom of a pole represented on the left photograph by Pl—Pz and on the right photograph by P1’—P2’ are (1/ + y’) and (a- + 19’), as in figure 23, and differential parallax between the top and bottom of the pole is (:11 + 11’) — (:0 + w’) or A — B. With double-projection instruments the vertical distance 01) is actually measured, rather than the horizontal distances that are measured on paper prints. The distance 01), measured generally in tenths of millimeters, is converted to feet by multiplying by the K factor, which represents the number of feet on the ground per 0.1 millimeter in the stereoscopic model. Points (7 and I) represent the intersections of those light rays that pass through the top and bottom of the pole respectively. 52 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING stereometer-type instrument or parallax bar is used because it permits more consistent readings of smaller increments of length than a ruler. Stereometers are generally constructed with a principal scale graduated in millimeters and a subordinate drum scale calibrated in hundredths of millimeters for recording parallax measurements; a few instruments are designed to record in inches. To make parallax measurements from aerial photo- graphs it is first necessary to orient the photographs properly for stereoscopic viewing. This is accom— plished by alining the photograph centers and conju- gate centers along a straight line—the equivalent of the flight-line direction—and separating the photo- graphs for comfortable viewing (fig. 25). Photograph centers are located by marking the intersections of lines drawn between fiducial marks at the opposite sides of each print. Each center is then transferred stereoscopically to the other photograph of the stereo- Conjugate image of left photograph center Conjugate image of right photograph center Left photograph center scopic pair and its conjugate image point marked. Parallax measurements are then made in the so-called w direction, parallel to the flight line. Measurement is accomplished by adjusting the separation of the dots of the parallax bar until a single fused dot, seen stereo— scopically, appears to rest on the ground at the first point selected. The instrument reading is recorded and the procedure is repeated for the second point selected. The difference in readings is the parallax difference between the two points. In making parallax measurements the fused dot will be seen readily when it floats above the apparent ground surface of the stereoscopic model but will appear to split into its two component dots as it is lowered below the ground sur- face. If the fused dot, as it is being lowered, appears to split at some point just above the ground surface, a slight change in the separation of the photographs may be necessary to permit simultaneous viewing of the floating dot and the stereoscopic model. Right photograph center Fiducial marks FIGURE 25.—Sketch showing correct orientation of photographs for stereoscopic viewing. MEASUREMENT , 53 To obtain the height of an object or difference in altitude between two points the difference in parallax must be related to the geometry of the stereoscopic model. For measuring heights from paper prints this relation is best expressed by a formula which, in its simplest terms, is h=%I-Ap (1) where h=height of the object, in feet, or difference in altitude being determined; H =height of airplane, in feet, above mean terrain (determined from specifications of photographic mission or relation H =f/S, see below); b=photobase (commonly determined by averag- ing the distances between the center and conjugate center of each photograph of the stereoscopic pair); units of measure may be millimeters or inches but must be the same as A1); and Apzparallax difference, in millimeters or inches, as determined with stereometer-type instru— ment (distance A—B of figure 23). It will be noted that as H increases, the measure of A1) for any given vertical interval will decrease; how- ever, the absolute value for any one unit of A1) will increase correspondingly. On the other hand, if focal length is increased and flying height remains constant the measure of A1) for a given vertical interval will increase but the absolute value for any one unit of Ap will decrease correspondingly. The above formula (1) may be used without any appreciable error if relief in an area is low; Ap will be small. If relief in an area is high one of the following two formulas, (2) or (3), should be used: I ”m“ (2) where h=height of the object, in feet, or difference in altitude being determined; H’zheight of airplane, in feet, above the lower of the two points whose parallax difference has been measured; ab=photobase adjusted to the lower of the two points whose parallax difference has been measured (commonly determined by measur- ing the distance between photograph cen- ters—O—O’ of fig. 23—and subtracting from it the distance between conjugate image points at the lower altitude—distance A of fig. 23); units of measure may be millimeters or inches but must be the same as Ap; and Ap=parallax difference, in millimeters or inches, measured with stereometer-type instrument (distance A—B of figure 23); ‘ H/ where h=height of the object, in feet, or difference in in altitude being determined; H’zheight of airplane, in feet, above the upper of the two points whose parallax diffirence has been measured; l ab=photobase adjusted to the upper of thic two points whose parallax difference has‘ been measured (commonly determined by measur- ing the distance between photograph cen- ters—0-0’ of fig. 23—and subtracting from it the distance between conjugate image points at the upper altitude!distance B of fig. 23); units of measure may be millimeters or inches but must be the same as A1); and Ap=parallax difference, in millimeters or inches, measured with stereometer-type instrument (distance AfiB of figure 23). ‘ Formula (2) is used when H’ and ab are determined by referring to the lower of the two points whose alti- tude difference is desired, as in measuring upward from the base to the top of a cliff. Formula (3) is used when H’ and ab are referred to the upper point, as in measuring downward from the top to the bottom of a clifl". Most commonly the lower of two points is selected as the reference point and measurements are made from that point to some higher point in the stereoscopic model; thus formula (2) is more widely used than formula (3). _ Parallax differences generally can be read wit-hm small tolerances (see p. 73), and errors in comput- ing differences in altitude result mainly in the determi— nation of flying height and photobase, assuming photo- graphs have little or no tilt. It is therefore generally desirable to determine flying height (H ’) and adjusted photobase (ab) carefully because a given percentage error in one of them may cause a similar percentage error in the computation of altitude difference. In addition, failure to use formula (2) or (3) when paral- lax difference (Ap) is large will also cause a consider- able error in the final result even though the a ‘justed photobase (ab) has been carefully determined. For Calculation of absolute heights as is nec ssary, for example, in determining stratigraphic thic ess or displacement on faults, the factor H’ may be ‘deter- mined from a base map of known scale or from other control using the relation H ’= f/S. If the altitude difference between some two points, A and B, is desired 54 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING and if neither A nor B falls in a convenient map posi- tion for sealing a distance and determinnig H ’ for one of them, it may be necessary to make parallax meas— urements with respect to some selected third point, 0, for which the flying height (H’) can be readily cal- culated. Where altitudes are to be used for computing angles of dip it is not necessary to know the absolute height of H’; the tangent relation of angles . vertical distance Tan angle Of dlp_horiz0ntal distance (4) involves only relative horizontal distances and relative differences of altitudes, and thus any value may be assumed for H’ when measuring parallax difference and determining the scale of the photograph to be used in computing dips. In photogrammetric terms this relation may be expressed as flying height (feet) X parallax . difference Tan angle 0f dip :horizontal distance on ground (5) (feet) >< adjusted photobase or Tan angle of dip=WI:b:—Zp—)- (6) Since flying height (feet) horizontal distance on ground (feet) is also equal to focal length (inches) horozontal distance on photograph (inches) the following formula may also be used to calculate angles of dip when flying height, H’, is not known or cannot be determined: focal length X parallax difference ( ) distance on photo—. graph >< adjusted photobase Tan angle of dipzhorizontal The tangent of dip, in photogrammetric terms, has been described in more detail by Desjardins (1943b, p. 1536—1538) and Elliott (1952, p. 8). When angles of dip are determined from enlarged photographs and formula (7) is used, the effect of photographic en— largement on focal length must be considered; effective focal length is increased in direct proportion to the amount of photographic enlragement (see p. 4). Hemphill (1958a, p. 43) has devised a chart that shows the number of feet represented by each milli- meter of parallax change as H’ and (a?) + Ag?) of for- mula (2) vary. The factor determined from the chart is then multiplied by Ap to arrive at h, in feet, pro~ vided formula (2-) is used. A simple circular photo- grammetric computer has also been designed that per- mits easy computation of the number of feet repre- sented by each millimeter or hundredth millimeter of parallax as flying height and photobase vary. In addi- tion, the computer allows multiplication of this factor by the parallax change to give altitude differences in feet. The number of feet represented by each hun— dredth millimeter of parallax measured on paper prints may also be determined from the ratio Flying height (feet) Photobase (millimeters) X 100' This relation is further discussed on page 73. Need for correct orientatiom of photographs.—Mis- orientation of a stereopair of photographs can be con— sidered from the standpoint of rotation around the cen- ter of one or both of the photographs. This rotation results in misalinement of the stereopair and flight line. Parallax differences are not then measured in the true as direction, or flight—line direction, but at a slight angle to it; an error in the adjusted photobase also results. The measurement of parallax difference is not normally significantly affected but the adjusted photobase on a stereopair misoriented by a few degrees, even though the human eyes can tolerate a certain amount of error and yet see a well—defined stereoscopic image, may be in error by as much as 5 to 10 percent. This will cause a similar error in differences in altitudes determined by the parallax method when paper prints are used. USE OF DOUBLE-PROJECTION INSTRULIENTS When double-projection instruments are used to de— termine altitude difference between two points, only one distance need be measured. This is a vertical dis— tance within the stereoscopic model that results from the translation of linear parallax recorded on the glass— plate diapositives to angular parallax in the viewing arrangement (fig. 24). The vertical position of inter- secting light rays to the lower point to be measured is determined by placing the floating dot on the apparent ground in the stereoscopic model and recording the instrument reading. The vertical position of the upper point is recorded in a similar manner. The difference in readings is a measure of the parallactic displacement and is related to the difference in alti— tude of the two points. With double-projection in— struments measurements are normally made in incre- ments of tenths of millimeters, although some instru- ments are so designed that selection of proper gear trains permits readings to be converted directly to heights in meters or feet. In stereoscopic models formed in double-projection instruments the vertical scale remains constant with MEASUREMENT“ 55 respect to the horizontal scale and a simple multiplica— tion factor—called the K factor——for relating incre- ments of vertical measurement to absolute feet can be determined by converting the standard fractional scale (representative fraction) of the model to feet per unit of vertical measurement, that is, per 0.1 millimeter. This is most rapidly accomplished using the formula _ model scale denominator *:%,048 (number of tenths of millimeters in 1 foot) K Thus a stereoscopic model with a scale of 1:4,000 would have a K factor of 4,000/3,048 or 1.31 feet per 0.1 millimeter of vertical measurement. Increments of vertical measurement are merely multiplied by the appropriate If factor to obtain differences in altitude in feet. A constant factor for converting increments of vertical measurement to feet is appropriate because the horizontal and vertical scales are equal and con— stant throughout the stereoscopic model, but when paper prints are used, the horizontal scale is not con— stant throughout the model and hence different multi- plication factors must be determined for different levels in the model. Because the horizontal and vertical scales in stereo- scopic models from double—projection instruments are equal and constant throughout the model, inclined dis— tances, such as stratigraphic thicknesses of dipping beds, may also be measured. A special device for measuring distances in inclined directions was devel- oped by R. H. Morris and C. L. Pillmore of the Geo- logical Survey; it consists of a multiplex tracing table modified so that the platen, platen socket, and light source can be tilted as a unit (fig. 22). An additional worm-screw drive, separate from the conventional worm—screw drive, allows the platen to move in an in- clined direction perpendicular to the surface of the platen. The resulting measured distance Within the model is converted to feet in the usual manner, and a measure of the inclined distance is obtained. TILT Tilt in present—day vertical photography is generally small—under good flying and photographic conditions 50 percent of the photographs taken for domestic mapping are reported to be tilted less than 1° and 90 percent tilted less than 2° (see Tewinkel, 1952, p. 319). But tilt may be significant in the photogrammetric calculation of many geologic measurements, and care must therefore be exercised in determining altitudes from parallax measurements. The significance. of tilt on vertical measurements used in geologic interpreta- tion depends in large part on the geologic problem. Where vertical measurements must be referred to a horizontal datum, as in structure contouring, tilt may cause significant errors, but where relative vertical measurements between pairs of points are desire l, as in constructing isopach lines, tilt may have relatively little effect especially where points of any one pair are sepa- rated by only a small horizontal distance see p. 73). If only simple streometers are available for parallax determinations it may be necessary to entirely discard some photograph prints unless (a) vertical control is available and the geologist carries out the tedious task of constructing a correction graph (Des- jardins, 1950, p. 2304—2305; McNeil, 1952, p. 610—615; and Visser, 1954, p. 849—853), or (b) points of meas- urement are selected so that the effect of tilt will be minimized (Hemphill, 1958a, p. 46—47, 49). It is also possible to partially correct for tilt locally on planar orientations, such as the strike and dip of beds, by stereographic projection and appropriate rotations in stereographic constructions. If photogrammetric instruments are available that allow photographs to be tilted, it is generally pos- sible to eliminate any large amount of tilt (greater than 1°) by careful inspection of physiographic fea- tures of the stereoscopic model, thereby perniitting reliable parallax measurements to be madei even though vertical control is not available for leveling the model. For example, lakes or other standing bodies of water may appear tilted, or certain screams or stream meanders may appear to flow uphil (fig. 109). Tilt is also often detected by observing that headwater tributaries in subdued divide areas iWhere gradients are very lOW appear to flow uphill. Atljust— ment of the instrument so that the lakes appear flat and streams have a normal gradient Will minimize tilt, and parallax measurements can then be made. For certain sensitive measurements, such as measure- ment of low stream gradients or measurements used to determine the strike of low—dipping beds, it is essen- tial that vertical control for leveling models be avail- able and that a precision plotting instrumentd such as the Kelsh plotter, be used. Because a small amount of tilt may seriously affect the measure of stream gradient the economic advantage of photogrammetric techniques may be lost where the measure of‘ gra— dients is the primary photogrammetric objective, in- asmuch as a considerable amount of vertical cdntrol must be obtained by ground survey to level the stereo- scopic models. However, for many geologic measure- ments, particularly where relative intervals are to be measured, ground control, although desirable, i not essential for making satisfactory parallax deterlffina— tions, either from paper prints or glass-plate} dia— positives. I 56 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING DETERMINATION OF ALTITUDE DIFFERENCES FROM A SINGLE PHOTOGRAPH Under certain conditions altitude differences between two points may be determined by making appropriate measurements of radial displacement on a single photo— graph and substituting them in the formula 1&1 7' h where hzdiflerence in altitude desired; m=relief displacement of upper image point with respect to the lower image point; H=height of airplane above lower image point; and rzradial distance from principal point of photo- graph to lower image point. The distance m and r must be measured in the same units. The height, h, will then be in terms of the units chosen for H. This procedure is only applicable to vertical objects where the upper point is known to be directly above the lower point. Under these restricted conditions, the length of the image of an object, such as a tree, or a cliff face at right angles to a radial line from the principal point, is a measure of the relief displace- ment. The radial distance to the lower point can be easily measured, and the flying height can be obtained from the formula H =f/S (see p. 53—54) when base maps are available, or from the specifications of the photographic mission. Thus, for example, if the ra- dial displacement of a cliff is 0.03 inch, the flying height of the airplane 10,000 feet, and the lower point is 3 inches from the photograph center, the height of the cliff will be h=0.03 x 10,000 3.0 h: 100 feet. Because relief displacement is generally small, a small error in measuring such a distance on a single photo- graph will result in a relatively large error in the height of the object, which further limits the useful— ness of the formula for determining vertical intervals. GEOLOGIC USES OF PARALLAX MEASUREMENTS Parallax measurements always result in the deter— mination of spot altitudes or vertical intervals when papers prints are used; and generally result in spot altitudes or vertical intervals when glass—plate dia- positives are used in double—projection instruments, except when a special tracing table is used that per- mits measurement of inclined distances (fig. 22). These spot altitudes and differences in altitudes com— bined with horizontal distances determined from the stereoscopic model, or under some circumstances from a. single photograph, may be used to compile struc- ture—contour maps and to determine dips of beds, thicknesses of beds, offsets on faults, gradients of streams, and related data. Because of relief displacement, however, it may be necessary to correctly locate the relative horizontal positions of points, Whose altitudes have been meas- ured, before computing strikes and dips, determining stratigraphic thicknesses, or any other geologic meas- urements Where dips are involved. Because tilt is usually negligible in present-day photography, hori- zontal positions of points generally may be plotted satisfactorily with any instrument that removes relief displacement, such as the radial planimetric plotter, paper-print plotters, and double-projection plotters even though the stereoscopic model cannot be leveled. Horizontal distances may then be scaled off. How- ever, in the absence of plotters for locating the rela— tive map positions of radially displaced points when measurements are made with parallax bar and paper prints, an overlay or similar procedure may be used to determine corrected horizontal distances, unless 0r- thophotographs (see page 69) are available from which correct horizontal distances may be scaled. The. overlay procedure requires first laying out on transparent material a line equal in length to the ad- justed photobase. The overlay is then placed over the right photograph of the stereoscopic pair so that the line drawn is coincident with the flight direction and its right end terminates at the photograph center. Radial lines are then drawn on the overlay from the photograph center through all points whose relative horizontal positions are to be determined. The pro- cedure is repeated with the overlay positioned over the left photograph, again with the original line co— incident with the flight direction and its left end ter- minating at the photograph center. The intersection of a pair of lines through the same image points is the corrected horizontal position for that point. DETERMINING STRIKES AND DIPS \Vhere bedding surfaces coincide with topographic surfaces it is generally sufficient to measure the alti— tude difference between only two points, one directly downdip from the other in calculating the amount of dip. The horizontal distance between these two points, together with the diflerence in altitude, gives the re- quired information for determining the angle of dip from the trigonometric relation Vertical distance ——.————.——— :- trlnorent of '11] ’18 di ). Horizontal distance ‘ b ‘ é 1 If relief in an area is low the horizontal distance may be scaled directly from a single photograph with- MEASUREMENT 3 57 out significant error in computing the dip, but where relief is moderate or high a correction for the relief displacement of the upper point with respect to the lower point generally should be made. In the unique circumstance Where the strike is radial from a photo- graph center or where the surface on which the dip to be measured is at or near a photograph center, there is little or no relief displacement in the dip direction, and no correction in scaling the horizontal distance need be made. Hemphill (1958a, p. 53) sug— gested that the corrected horizontal distance between the upper and lower points should be at least 0.2 inch for computing dip angles. Where dips are greater than 50°, a longer horizontal distance is needed to determine the dip angle reliably, as the tangent increment per degree of dip is significantly greater for steep angles of dip than for low angles of dip. The strike line generally can be determined readily by inspection of the stereoscopic model and noting two points of equal altitude on a bed. Where dips are low, however, tilt in the photographs will affect the direction of strike: the lower the dip, the greater the effect, generally, on the amount of shift in the azimuth of the strike line. Graphs may be constructed to show the effect of tilt 0n low—dipping beds. (See Hemphill, 1958a, p. 48.) Where a bed crops out in a valley wall it will gen- erally be necessary to determine the altitudes of three points on the bed. In areas of moderate or high re- lief corrections for relief displacement must be made before distances between these points can beldeter- mined; otherwise distances may be scaled directly from the single photograph. Determination of the strike direction and amount of dip are then made graph- ically or graphically and trigonometrically. DETERMINING STRATIGRAPHIC THICKNESS In areas where beds are horizontal or nearly hori- zontal the stratigraphic thickness may be detertmined directly by converting to feet the parallax difierence between the top and the bottom of the bed. ‘0 cor- rection is necessary for relief displacement. HOJWever, if beds are inclined, the angle of dip must first be determined; then corrections must be made fori relief displacement and for the effect of dip on the btrati- graphic thickness. The thickness may be calctllated by simple trigonometry or by graphic solution (fig. 26). Desjardins (1950, p. 2308—2309; 1951. p. 829—830) suggested that floating lines be employed with jpaper prints for determining stratigraphic thicknesses lwhere beds do not dip more than 15°. In the “floatin line” procedure, lines are drawn on transparent strps of FIGURE 26.—Diagram of gently dipping: beds showing relation of stratigraphic thickness to differential parallex determimd at any two points along dip direction and at the formation contacts. Stratigraphic thickness is determined from the formula t=x+x’= where t: stratigraphic thickness; h d + cos 0 sin 0 h=difference in altitude between some point on the lower contact of the bed and some point, along a line at right angles to the strike line, at the upper contact of the bed; altitude h is determined from the parallax formula; ‘ d=corrected horizontal distance between points at lower and upper contacts of the bed; and 0=angle of dip. 58 material that are superposed over each of the photo- graphs. When one line over one photograph is posi— tioned parallel to a line over the second photograph, a single floating line will appear when viewed streo- scopically unless the lines are parallel to the flight direction. The line will float above or within the model and appear to be horizontal; its vertical posi- tion in the model will depend on the separation of the two individual lines in the flight direction. A series of lines will form a grid or reference plane (Smith, H. T. U., 1943a, p. 171; Desjardins, 1943a, p. 219; Hackman, 1957, p. 593). When any one pair of lines is divergent rather than parallel the floating line in the stereoscopic model will appear to plunge. By changing the horizontal separation of any two lines in the flight direction the floating line can be made to rise or fall in the model, just as with the floating dot of the parallax bar. When the floating-line pro- cedure is used in measuring stratigraphic thicknesses, a line is first floated preferably in the bedding plane at the upper formational contact, and parallax meas— urements are then made at the lower formational con- tact and on the floating line at a position in space vertically above this point (see fig. 27) If beds are AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING horizontal or nearly horizontal the parallax difl’er— ence will be approximately a measure of the strati- graphic thickness. If the beds dip, the stratigraphic thickness will bear a cosine relation to the dip angle and parallax measurement (see fig. 27). Where dips are steep it may be desirable to measure a horizontal distance along a line at right angles to the strike direction between two formation contacts and relate this distance and dip angle to stratigraphic thickness. Points must be chosen at approximately the same altitude in the stereoscopic model (fig. 28). ISOPACH lMAPPING The use of aerial photographs in studies of certain areas in the Colorado Plateau of Western United States has included the photogrammetric compilation of isopach maps that show local thickenings of certain formations in which uranium minerals are likely to occur. These thickenings of formations are channel— fill deposits in stream channels that were cut in the underlying formation. Uranium minerals are locally concentrated in these deposits. In the Monument Val— ley area of southern Utah it has been demonstrated that isopach lines, indicating the trends and thick- nesses of different channel—fill deposits, could be drawn Floating line in stereoscopic model N. . / 5/ / | l I ll? .- 1 / // / FIGURE 27.~Diagram of gently dipping beds showing relation of stratigraphic thickness to differential parallax determined by floating—line method. Stratigraphic thickness is found from the formula where tzstratigraphic thickness; h=difference in altitude between bottom of bed and point on floating line vertically above, as determined from the parallax formula; and 0=angle equivalent to angle of dip. MEASUREMENT 59 on the basis of measurements made on vertical aerial photographs. In this area uranium minerals are pres- sent in channel-fill deposits at the base of the Shina- rump member of the Chinle formation of Triassic age. However, because the top of the Shinarump member is eroded away in many areas, or because it is grada— tional with the overlying member of the Chinle formation, a stratigraphic unit below the Shinarump member was chosen as the unit for which isopach lines were drawn. A thinning of this underlying unit then was interpreted to indicate a thickening of, or channel deposits of, the overlying Shinarump mem- ber. The unit selected for isopach measurements, the Moenkopi formation, is expressed on aerial photo- graphs as a dark-toned slope-forming unit that con— trasts markedly with the light-toned cliff-forming units of both the underlying De Chelly sandstone member of the Cutler formation and the overlying Shinarump member of the Chinle formation (fig. 44). A series of altitude measurements was then made along the contact of the Moenkopi and Shinarump formations from aerial photographs by means of the Kelsh plotter, and the locations of these measure— ments were plotted on a base sheet. Similarly, a series of altitude measurements was made along the contact of the De Chelly and Moenkopi formations and the locations were plotted on the base sheet. Measure— ments along the respective geologic contacts were generally spaced at horizontal distances of about 500 feet. Because the stratigraphic units dip locally it was necessary to compute the strike and dip of the beds and to correct for the dip angle in determining the stratigraphic thicknesses of the Moenkopi interval. Thickness computations were plotted at the points of altitude measurements along the contact of the Moen- kopi and the Shinarump, and isopach lines were drawn at 10-foot intervals on the basis of the distri- bution of thickness figures. The relatively large num— ber of readings, both for strike and dip of beds as well as for altitude measurements, permitted elimination of certain computations that were inconsistent with regard to the overall mass of statistical data. The locations and depths of channels of Shinarump mem- ber as shown by isopach lines based on photogram- metric measurements and plotting were generally in close agreement with those determined by field meth- ods. Details of the photogeologic study are described by VVitkind, Hemphill, Pillmore, and Morris (1960). FACIES CIIANGE Measurements from aerial photographs in conjunc— tion with stratigraphic studies of Cretaceous rocks in northern Alaska have demonstrated the rate and direc- tion of “shaling,” or facies change, of certain forma- tions. In the Utukok—Corwin area resistant rocks of the Kukpowruk formation, which is predominantly silty shale, siltstone, and sandstone, overlie less resistant FIGURE 28.~Diagrain of steeply dipping; beds showing relation of stratigraphic thickness to dip angle and horizontal distance between top and botton of bed. Stratigraphic thickness is determined from the formula I: where l=stratigraphic thickness; d sin 0 d=horizontal distance at right angles to strike line between points at same altitude on top and bottom of bed; and Ozangle of dip. 60 rocks of the Torok formation, which is largely clay shale, claystone, and silt-y shale. The Torok formation commonly forms the lowlands, which are characterized by rather uniform and low-dipping topographic slopes. The Kukpowruk formation forms high, resistant ridges that can be traced many miles. The contact of the two formations is marked by a break in topography that has been mapped in the field in conjunction with the study of aerial photographs. Slopes below the contact are generally smooth and unbroken; slopes above the contact are marked by conspicuous topographic breaks caused by hard, resistant sandstone beds in the basal part of the Kukpowruk formation. Vertical exaggera- tion of the stereoscopic model accentuates these topo- graphic breaks in slope. Photogeologic and field stud- ies Show that these resistant beds pinch out usually in an easterly or northerly direction (see fig. 97). It is significant that their topographic expression is lost. Ground observations along stream cuts indicate that the resistant beds grade laterally into shaly sections. Quantitative studies from aerial photographs of the amount of section affected by facies change in the Utukok-Corwin area show that several thousand feet of sandy section: of the Kukpowruk formation grade laterally to the east and north into shale of the Torok formation. Chapman and Sable (1960) stated that the methods used in these studies included tracing on ver— tical aerial photographs a resistant unit within the Kukpowruk formation around the flanks of a large open structure and then measuring the stratigraphic intervals between the resistant bed and the contact of the Kukpowruk and Torok formations to determine the new relative stratigraphic positions of the contact in different parts of the area. Photogrammetric meas- urements were made whenever possible in localities of good field control; a few stratigraphic thicknesses were determined solely by photogrammetric methods with- out field control. Locations for measurements were chosen along east-west and north-south lines. Relative stratigraphic positions of the contact of the Kukpow— ruk and Torok formations were then plotted graphi— cally against lateral distances between points of meas— urement; this resulted in a line whose gradient showed the average rate of rise of the contact between points of measurement. An average eastward—rising gradient of 58 feet per mile and an average northward-rising gradient of 97 feet per mile were calculated. It was concluded on the basis of plotting these aver— age gradients as vector quantities that a resultant maxi— mum gradient of 115 feet per mile exists and that more than 10,000 feet of sandy beds grade into shaly sections over the area studied. The maximum gradient direction was further interpreted to lie at right angles AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING to the direction of minimum facies change, which is believed to indicate the general trend of the old shore- line during the period of deposition of the Kukpowruk and Torok sediments. Thus the photogeologic meas- urement of stratigraphic intervals not only permitted determination of the directions and amounts of facies change, but further allowed inferences concerning the geologic history and environment, which may be significant in the search for petroleum in that area. STRUCTURE CONTOURING Structure contours are usually constructed on the basis of a series of spot altitudes, some of which may be photogrammetrically determined at the top of the horizon to be contoured, but many of which will be measured at formation contacts above or below that horizon. Photogeologic procedure is similar to field procedure and requires adjusting the altitudes of all points not on the horizon to be contoured by making appropriate considerations of strike and dip and strati— graphic thickness. Structure-contour lines are then adjusted to the resultant series of altitudes projected to the same formation contact. NOTOM—15 QUADRANGLE In photogeologic interpretation and mapping of the Notom-15 quadrangle, Utah, (Hackman and Tolbert, 1955) altitudes for structure contouring were measured using a Kelsh plotter. No previous geologic or topo— graphic mapping had been done in the area; geologic data and altitude measurements were plotted to a con— trol net established by photogrammetric methods. The Notom—15 quadrangle is well—exposed canyon country typical of the Colorado Plateau of Utah. Gently folded sedimentary rocks of Triassic and Jurassic age underlie the area. Canyons transecting the general structure trend made the application of photogeologic procedure-s ideal. A series of altitude measurements were made along the contact of the VVingate sandstone of Triassic age and the overlying Kayenta formation of Triassic( ?) age. The wide exposure of this contact throughout the area permitted sufficient measurements to control structure contouring. Structure contours were then drawn on the top of the VVingate sandstone. Subsequent fieldwork corroborated the general struc- ture as contoured by Kelsh plotter from aerial pho- tographs and also corroborated the distributions of rock formation as interpreted and mapped from pho- tographs. Positioning of other planimetric data, such as streams, was shown to be in excellent agreement. with the subsequently compiled standard topographic map of the quadrangle. The positioning of data and altitude determinations by Kelsh plotter thus resulted in a highly reliable geologic map of the area. MEASUREMENT 61 DISCOVERY ANTICLINE Procedures used in contouring the Discovery anti- cline in northern Alaska included stereometer meas— urement of altitudes combined with simple trigo— nometric computations and graphic constructions to compile a generalized structure-contour map (Marshall and Rosendale, 1953). These procedures contrast markedly with the Kelsh-plotter compilation of the N0tom-15 quadrangle, but demonstrate that simple methods may be useful in obtaining structural infor- mation of a reconnaissance nature from aerial photo— graphs. , Discovery anticline, a gently folded structural fea— ture approximately 25 miles long and 7 miles wide, is underlain primarily by Cretaceous; rocks. The area is generally covered with tundra grasses, but resistant beds within the stratigraphic section are expressed topographically as ridges, or breaks in slope, that can be traced for many miles; some beds are expressed by photographic tone due to differences in vegetation (see fig. 40). Dip slopes rarely coincide with topographic surfaces. Procedures for obtaining and positioning data for structure contouring involved the arbitrary selection of cross-section lines normal to the general structural trend. Strikes and dips of beds were then determined by simple stereometer methods at numerous localities and projected to the nearest cross—section line. Where the strike line did not intersect the line of section at right angles the apparent dip was determined and plotted on the section line. This procedure was fol~ 12° 15° 20° Top of marker bed lowed for all strikes and dips, and resulted in cross— section lines along which the different dips were plotted. A marker bed was then selected and its pro- file along the line of section graphically reconstructed on the basis of the positions and amounts of dip previ~ ously plotted (fig. 29). It was assumed that bedding throughout the stratigraphic section was parallel. Structure contours were then positioned along each line of section by further graphic constructions (fig. 30). However, the datum for each line of section commonly was different, depending on which part of the stratigraphic section was expressed in any one local area, and it was necessary to obtain stereometer measurements of altitudes between cross-section lines so that structure contours, as positioned along each line of section, could be tied together (fig. 31). The resulting reconnaissance map, plotted on an uncon- trolled photomosaic, indicated the general magnitude and closure of the anticline and the attitude and steep— ness of bedding. Figures 29—31 show graphic con- structions in reconnaissance structure contouring from measurements made with stereometer—type instruments. DETERMINING DISPLACEMENTS 0N mums The vertical component of displacement on some faults may be readily determined from parallax meas- urements made on marker beds on opposite sides of the fault by simple conversion of parallax difference to feet or other appropriate unit of measurement. If the fault plane dips, relative altitudes on marker beds may be related to the down—dip component of displace- ment by trigonometric or graphic solution. 27° 24° Line of section ’ a b c d 6 FIGURE 29.——Diagram showing graphic reconstruction of marker bed along line of section. Dips determined from aerial photographs are first plot— ted along line of section. The marker bed is then recon- structed by extending bed upward from point a at 12° dip for half the distance between a and I). From this point the bed is extended at 15° dip to a point half the distance between D and c. From this point the bed is again extended at 20° dip to a point half the distance be- tween 0 and d, and so on. A smooth curve is then drawn to represent the marker bed. G3 [\D AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING ._l E m 400' LLJ +— E n: , :> 300 o '— z O ‘3 , w 200 n: :3 ’— o g 100' ' ’— <0 HORIZONTAL LINE OF SECTION FIGURE 30.—Diagram showing graphic construction in positioning structure-contour lines on top of marker bed and the projecting of these lines to their relative horizontal positions along the line of section. 250’ 200’ 150' 100' A 8 x/ 1650’ \‘ 300: 250' 1600’ 1550' \k—I/ 1550' 1500' w 1500' \W \ 1400’ M 1450’ 50' \ 1400' 1350’ FIGURE 31.—Diagram showing relation of structurecontour positions along two lines of section at difierent altitudes. Relative positions of 50noo-32wluwm 553% un_01_m NEE. 30m .mw :8 .mw 10h .mw sow omm .mv Q. m; 0. 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