AERIAL PHOTOGRAPHY 



731 



Flying for the ultimate purpose of compiling contour maps must be done with the 

 utmost precision. The flying pattern is fundamental in the ultimate economy of the 

 job, as costs are a direct function of the number of pictures involved. 



P'ilm which is to be used for contour plotting must be handled with special care in 

 development. The emulsion should not be swabbed with cotton, as this introduces 

 the danger of slightly moving small sections of the emulsion which may not be firml}' 

 adhered to the base. This film should be dried slowly in a room where comparatively 

 high humidity prevails, and the film should be supported hanging free from a wire 

 with at least one support for each foot of film. 



Most contour maps are plotted from glass diapositives which should be made 

 from the film either during the period of the first 48 hr. after development or after 

 3 weeks have elapsed. Between these periods most types of film are at maximum 



Fig. 18. — The aerocartoKriiph is one ot the older types ol machines for plotting contours Irom 



aerial photographs. 



instability. The film itself, which is used for the original pictures, must be of the 

 low-shrink base type, which has been subjected to special conditioning to minimize 

 nonuniform shrinkage and expansion. 



A number of different methods are available for producing contour maps from 

 aerial photographs. The methods are all based upon the principle of stereoscopic 

 Adsion or the somewhat equivalent flicker method. Precision contour maps can be 

 plotted from these aerial photographs only by highly trained experts using elaborate 

 equipment, such as the stereoplanigraph, aerocartograph, or midtiplex. 



Bibliography 



Perodicals: 



Map Compilation from Aerial Photographs, Topographic Instructions of the U. S. Geol. Survey, Bull. 



788, Appendix F, 1928. 

 AcKLAND, F. W.: Use of Aerial Photographs for Mapping, Canadian Topnoraphira/ Surrey, Bull. 62, 



1932. 

 Air Corps Aerial Photography, War Dept., Traininq Manual 2170-6. 



