The use and evaluation of air photographs is a comparatively recent development in earth sciences. It is closely connected with advances of aeronautics in the last four decades and photography from air planes. However, utilization of air photography is not confined to earth sciences alone. Its widest use is in constructing accurate planimetric and topographic maps, made by air0triangulation and photogrammetric procedures. This science of photogrammetry is closely affiliated with the use of air photographs because its aim of constructing a map serving varied special purposes is done by photogrammetric methods.

The use of photogrammetry lies principally in interpretation of features visible on the air photograph. Since an air photo will disclose every surface feature which appears on a photo, the interpretation is highly specialized. Geology, engineering projects, soil science and utilization, geography, forestry, archeology and military planning make wide use of air photographs, though the interpreted features vary with each purpose. Certain branches of earth science like minerology, paleontology and geophysics are not applicable to air photo interpretation.

The photo interpretator (according to Summerson) needs a broad and varied background, because his objects and observations naturally fall into many different branches of science. He has to observe the problems of applied terminology, interrelation of sciences such as geology, climatology, botany and agronomy, geomorphology or history of land forms, and he must have the knowledge of principles concerning interpretation and prediction in natural sciences. He must also be somewhat of a polyhistorian, or man of all trades. The problem of identifying features on air photographs is foremost, and it is a difficult one. Setting up categories, i.e., "keys" or "typical" features is limited to specific circumstances under which the features develop.

There is always a gradation between categories and a change of forms and combinations which makes separating and following features uncertain. Interdependence of interacting elements like wind, water, ice, rain or vegetation cover on rocks, involve some fundamental knowledge of such sciences as physical geography, plant ecology and climatology, besides a solid foundation in geology and geomorphology. This stands for all special forms of photo evaluation if the end result is to construct maps for mining, petroleum prospecting, engineering or agrogeology.

Knowledge of the history of a terrain is also important because changes and movements of the earth's crust, like erosion or accumulation, must be understood. Thus, to rely on a straight identification key without knowing the history of the area will be of little help.

An interpreter of all stages of experience, sees in a photograph only what he actually is able to observe. It sounds paradoxical, but a beginner often sees more and comes to conclusions faster on a stereo model than an experienced interpreter who can distinguish between significant features and who knows that the last word always comes from the controlling field party, a necessary procedure in every branch of photo interpretation. Beginners and professionals should be skeptical of empirical identifications until the field observations agree with them.

The problem of "keys" is much disputed. It concerns the illustrative material of this book and, therefore, must be more fully discussed. Keys . . .

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