AERIAL PHOTOGRAPHY 729 



to print is thus established and recorded, an over-all measurement is made between 

 two known control points. A factor is then worked out for the enlargement or reduc- 

 tion of the entire strip of pictures to get the correct over-all distance with corrections, 

 print to print, according to the perpendicular measurements. This method breaks 

 down badly with tilted pictures or in case of much change in the elevation of the 

 ground. Its principal advantage is its simplicity of use. The results derived from 

 the use of this method do not compare in precision with the radial-control method, 

 but the method is cheaper and faster and requires less experience to operate. 



Contour Mapping. — Compilation of contour maps for aerial photography is the 

 most advanced and precise stage of the art. The aerial camera which is employed 

 must be a precision instrument, equipped with a lens which is sensibly free from dis- 

 tortion. The lens must be very carefully mounted in the camera in accordance with 



Fig. 15. — Standard single-lens camera installation. Photographer is looking into 

 view finder above his hand. ^ On the side of the cabin is the intervalometer for automatic 

 camera operation. 



the manufacturer's data. It is desirable to have the lens mounted by the manu- 

 facturer in a barrel, which is installed in the camera intact. This will insure that the 

 internal principal ray is a continuation of the external principal ray or at least as close 

 to this condition as the manufacturer with his factory facilities can produce. The 

 focal plane of this camera must be exactly perpendicular to the lens axis and the 

 collimation marks in the focal plane must precisely indicate the intersection of the 

 principal ray with the focal plane. Everything about the camera must fit rigidly, 

 so that, in case of disassembly or reassembly, parts will fall back precisely in the same 

 position. Interchangeable magazines are not tolerable in a precision camera unless 

 the magazine is doweled to the camera body and provided with elaborate provisions 

 for maintaining the focal plane constantly in its required position. This camera must, 

 of course, be equipped with a between-the-lens type of shutter and pictures should be 

 exposed at J-f 50 sec. or faster to get the maximum of sharpness. 



