AERIAL PHOTOGRAPHY 715 



3. Hangar and service facilities. It is desirable but not necessary to select a base 

 where the airplane can be kept in a hangar, and it is, of course, essential to determine 

 that an adequate supply of suitable grades of gasoline and oil are available. Airplanes 

 left on an open field should be staked down, and a watchman should be on duty with 

 the ship. 



Considerable study should be made of the type of terrain which is to be mapped. 

 There is some choice in the film which is to be used, and this will depend upon the 

 characteristics of the terrain and the camera. The film base maj^ be either standard 

 or topographic low-shrink base, depending on the purpose of the photography. Ordi- 

 narily three filters are carried on mapping projects, viz., Al, A2, and Minus Blue. 



If the flying is to be done at an elevation of 12,000 ft. or higher, the crew will per- 

 form much more efficiently if supplied with auxiliary oxygen. One suitable form of 

 oxygen is a mixture of 95 per cent of oxygen and 5 per cent carbon dioxide, which is 

 available at any oxygen house. The tank should be equipped with a metering device 

 so that the flow of oxygen can be regulated, and tubes should be made available for 

 both the pilot and photographer with ordinary pipe stems at the end of the tubes. 

 There are many other methods of supplying oxygen, and the above is merely one which 

 is in very general use. If a considerable amount of flying is to be done above 16,000 

 ft., the oxygen is absolutely essential. Mapping above 25,000 ft. should not be 

 attempted because the reduction of pressure at these altitudes is apt to seriously impair 

 the health of the operators. 



Flying for Oblique Pictures. — Before starting off to take oblique pictures, the 

 pilot should familiarize himself with the prevailing altitude requirements of the 

 Department of Commerce and local ordinances such as are in effect in many com- 

 munities. Before an oblique flight is attempted, a careful analysis of the subject to be 

 photographed should be made. Buildings, for example, having east exposures should 

 be shot in the morning, south exposures at midday, west exposures in the afternoon, 

 and north exposures preferably in the summer season early in the day. 



The composition of the picture must be given a proper amount of thought. For 

 example, most pictures are more pleasing if taken at a fairly flat angle. Thus, if the 

 city ordinances require that an altitude of 1500 ft. be maintained and if it is decided 

 that a suitable angle is 45° from the horizontal, the nearest that the airplane can get 

 to the subject is the hypothenuse of a triangle which is 1500 ft. on each side, which is 

 about 2200 ft. away. The size of the image can now be determined. The scale of the 

 image is arrived at by the following formula: S = D/f. This means the scale equals 

 the distance awaj^ D, divided bj'- the focal length /. Thus, if a 12-in. focal length is to 

 be used at a distance of 2200 ft., we have: 



^ = 2200^^2 = 183 ft. per in. (1) 



In other words, an object on the ground which is 183 ft. long, at right angles to the 

 lens axis, will appear 1 in. long in the photograph. Consequently, a fairly large 

 building under these conditions is going to occup.y only a small fraction of the total 

 picture. Thought should be given as to the composition of the rest of the picture, 

 i.e., how much skyline to include, etc. It is customary when taking a small object- 

 like a building to subsequently enlarge that section of the picture which contains the 

 building. Conversely, the aerial photographer is frequently called upon to photograph 

 subjects which are so extensive that when the picture is taken from a distance great 

 enough to include the entire subject, the detail becomes disappointingly small. Thus, 

 for example, if pictures are required of a section of a city with one mile in the fore- 

 ground, the picture may be so taken that the mile occupies 6 in. of image. Thus the 

 scale of the foreground of the picture will be 



5 28Qg = 880 ft. per in. (2) 



