714 HANDBOOK OF PHOTOGRAPHY 



lens cameras also have advantages in contour mapping under some conditions. These 

 cameras may cover a very wide angle, so that the terrain photographed in a single 

 composite exposure may have dimensions four or five times greater than the airplane 

 altitude. While such pictures are usually inferior in photographic quality to single- 

 lens pictures, they have speed and economy on their side and are quite extensively 

 used throughout the world. 



Personnel. — There are fewer really competent photographic mapping pilots than in 

 almost any imaginable type of occupation. Out of 25 pilots who would be rated as 

 excellent, by all the ordinary criteria, only one on an average will develop into an 

 excellent pilot for aerial photography. While practice will improve the performance 

 of almost any pilot to a certain degree, the main qualification seems to be something 

 instinctive over which a man has little control. It has been found that a great many 

 pilots can successfully map over so-called "sectionized country," i.e., country like the 

 middle west where the roads and fences all run true north and south or east and west. 

 These serve as a guide and greatly simplify the problem of the pilot. The moment the 

 pilot starts over unsectionized country, particularly if he does not have good maps 

 available, he is in trouble. Very little can be said, in an article of thisk ind, which will 

 help the pilot; the difficulties are being enumerated merely as a warning to inexperi- 

 enced people who may underestimate the difficulties encountered in this phase of 

 aerial surveying. 



The aerial photographer should be familiar with general photography and should 

 be at home in the air. He must be able to estimate accurately his light conditions and 

 must be experienced in determining the proper time interval between exposures and in 

 turning the camera for the proper angle of crab. 



Organizing the Photographic Mission. — In approaching an aerial-mapping project, 

 a proper choice of airplanes and cameras must first be made. In addition to the air- 

 plane qualifications listed heretofore, the airplane should have a capacity to maintain 

 an altitude considerably higher than the altitude at which it is going to be called upon 

 to work. Otherwise it will prove unstable. 



Similarly, a suitable camera should be selected, and the question as to whether a 

 manually operated or an automatic camera is to be used may depend upon the avail- 

 ability of equipment and the extent of the project. While manually operated cameras 

 are thoroughly practical, experience has proved that automatic cameras, which run 

 continuously at any desired interval, leave the operator more time to level the camera 

 and correct the crab. 



Weather study is of primary importance in planning the photographic mission. 

 Within the United States weather averages will run all the way from less than one 

 mapping day per month, at certain seasons of the year, to as high as 25 days per month. 

 Obviously, this is the most important cost factor. Weather information can generally 

 be secured from the nearest U. S. Weather Bureau Office, and nation-wide weather 

 averages, as they pertain to aerial mapping, are available through studies which 

 have been made by the Soil Conservation Service and the Agricultural Adjustment 

 Administration in the U. S. Department of Agriculture. 



In selecting the base from which the airplane is to operate, the following factors are 

 important : 



1. Communications. It is essential to get the exposed film quickly to the labora- 

 tory so that it may be checked, to avoid recurrence of anj^ trouble. 



2. Choice of airport. The airport should be preferablj'^ close enough to the area 

 to be mapped so that the area may be reached during the period the airplane is climb- 

 ing. Otherwise flying time is wasted each time the airplane goes out and comes 

 back. If possible an airport should be selected at which United States weather reports 

 are available. 



