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PHOTO INTERPRETATION IN EARTH SCIENCE 
plane and hand camera for supplementary photography. A need has been 
expressed for the following additional equipment not currently available: 
(1) a device for rapid and accurate dip determination on strata; (2) some reliable 
means for eliminating or for automatically recording the tilt of the photo; and 
(3) a direct method for drawing topographic profiles. 
In photo interpretation, instrumental equipment is subordinate in impor- 
tance to the operator's mental equipment. In many types of interpretation, 
the quantitative aspects are overshadowed by the qualitative, and for the 
latter the primary requisites are sound professional training and experience, 
coupled with keen powers of observation and rigorous faculties of analysis 
and synthesis. No amount of instrumentation can take the place of human 
eyes and brains. 
The procedures in photo interpretation are comparatively simple in principle, 
but frequently are of infinite complexity in practice. In many situations, inter- 
pretation is as much of an art as a science, drawing upon a broad background 
of knowledge and experience, and requiring a fine cultivation of the powers 
of judgment. : 
From the mechanical standpoint, interpretation consists basically of identify- 
ing rocks, surficial deposits, and soils, tracing outcrops, measuring dips, and 
endeavoring to synthesize the whole geologic picture from its more or less 
separate components. Generally the results of observation and inference are 
recorded by annotation of the photos in color, and this is followed later by 
transfer of the essential data from the photos to a base map. 
The criteria used for identifying the desired geologic features are many 
and varied. In general, they are the ones long familiar to the field geologist, 
although applied from a different perspective. The shape, size, distribution, 
and orientation of individual landscape elements, both major and minor, are 
naturally important. In many places, vegetation is indicative of soil and sub- 
surface drainage conditions, and the distribution of vegetation provides a clue 
to the occurrence of hidden geologic features. The over-all pattern of landforms, 
drainage features, and vegetative zones reflect broad geologic trends, and criti- 
cal analysis of local details, particularly where these seem anomalous or dis- 
cordant with the broader pattern, frequently gives a clue to important geologic 
features, such as structural traps for oil. All of these criteria are familiar to the 
well-trained scientist, but can be used far more effectively and rapidly on 
photos than on the ground or on maps. Perhaps the only types of criteria which 
are peculiar to photos are such things as color tones and textures, which symbo- 
lize various phenomena not directly discernible on the scale of the photo, and 
with the monochromatic limitations of the photo. 
In carrying out the interpretation procedure, the interpreter works from the 
better known to the less known or unknown, in so far as possible. He first en- 
deavors to correlate photographic detail with all available data on the surface 
geology of the area studied, or of the nearest comparable area. He consults 
topographic maps, geologic maps and reports, soil maps, drilling records, 
ground photos (both black-and-white and in color), geophysical traverses, and 
any other types of information to which he has access. This stage of the work 
aids in establishing criteria for subsequent study. 
In undeveloped or unexplored territory, the above aids may be lacking, 
must rely wholly on his basic knowledge of his subject, 
manuals, or interpretative ‘keys’ as 
jr the latter has been growing, and 
mains to be done to set up adequate 
and the interpreter 
supplemented by such generalized books, 
may be available. Realization of a need fc 
some progress has been made, but much re