CHAPTER XXIV 

 AERIAL PHOTOGKAPHY 



By Leon T. Eliel 



Introduction. — There are two possible objectives in taking aerial photographs: 

 (1) to produce a picture; (2) to make a map. 



We draw a clear line of distinction between the use of this material for picture and 

 for map purposes. The picture is a product which is to be looked at, to be used for its 

 artistic and visual value and for its commercial applications where nothing but the 

 photographic impression is required. 



Maps compiled from aerial photography may also have photographic or visual 

 value, but they are primarily maps made to scale for technical uses. 



Two kinds of aerial photographs are taken to be used for such visual information 

 as can be derived from them. The most commonlj'' used is the oblique photograph, 

 which is so termed because the camera axis is held obliquely at the time of exposure 

 (Fig. 1). Most aerial photographs which are used for illustrative purposes are of this 

 type. 



The second type of aerial photograph used for visual purposes is the vertical photo- 

 graph (Fig. 2). This photograph approaches being a map, although, because of 

 various inherent defects and distortions, it cannot be considered as a map until it has 

 been processed for this purpose. 



Oblique photographs have a variety of uses including advertising; progress pic- 

 tures; reports; collateral information to illustrate maps; recording emergencies such 

 as floods, earthquakes, etc.; city planning uses for zoning, building counts, rate litiga- 

 tion, etc.; and a variety of other subjects. 



Vertical photographs are extensively used for geological study, timber cruising, 

 city planning, and right-of-way work. The most extensive recent use has been on the 

 part of the Agricultural Adjustment Administration of the United States Department 

 of Agriculture for checking contract compliance on the part of farmers operating under 

 AAA benefits. 



These vertical photographs are suitable for viewing stereoscopically in relief and, 

 when properly handled in conjunction with certain measurements made on the ground, 

 can render information of reasonable accuracy. 



Maps compiled from aerial photographs may be of a number of kinds: 



1. Photographic mosaic maps 



2. Line maps: 



a. Planimetric maps (Fig. 3, page 710) (showing detail projected on one plane) 

 6. Contour maps (Fig. 4, page 711) (showing elevations in addition to 

 planimetric detail) 



Details of most of the applications broadly noted above will be enlarged upon later 

 in this article. With the general scope of aerial photography thus in mind, we can now 

 proceed with some detailed considerations of the technique involved. 



Requirements of Aerial Photography. Airplane. — For oblique photography, 

 almost any kind of an airplane that is big enough to carry a pilot, a photographer, and 



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