AERIAL PHOTOGRAPHY— BAISLEY 389 



from a slightly different position. As these pictures are taken from 

 different positions, there results an apparent displacement of objects 

 of different elevations in one picture when compared with the same 

 objects in another picture. From these displacements the differences 

 in elevation between different objects is determined mechanically 

 when such overlapping pictures are observed in stereoscopic plotting 

 machines such as the multiplex or aerocarti graph. 



The multiplex (pi. 10, fig. 1) is a plotting machine in which sepa- 

 rate projectors similar to the ordinary magic lanterns used to amuse 

 children are held on a frame work in a position which simulates that 

 occupied by the aerial camera at the instant the picture was made. 

 By means of a simple optical device the operator sees with one eye 

 one picture projected on his drawing board and with the other eye 

 the overlapping adjacent picture. The result is a visual model in 

 three dimensions which can be rapidly drawn as a contoured map. 

 The aerocartigraph (pi. 5, fig. 2) is a more complicated instrument 

 for accomplishing the same end and produces results of a considerably 

 higher degree of accuracy. 



The rapidity with which aerial mapping can be accomplished is 

 best shown by examples. In the summer of 1936, in order to study 

 all the area within a radius of 18 miles of the center of Washington, 

 the photographic detachment at Boiling Field, D. C, was told 

 to prepare a mosaic map 36 miles in diameter with the center of 

 the District of Columbia as its center. This area was photographed 

 with one airplane during two consecutive days, requiring only 11 

 hours of flying to photograph the entire area. All the laboratory 

 work, consisting of developing the film, printing, laying, and copy- 

 ing the mosaic was completed within 10 days, and copies of the 

 mosaic map (pi. 11, fig. 1) were delivered within that time. This 

 was all single-lens camera work, done with an 8i/4-inch focal length 

 K3-B camera from an altitude of 15,000 feet. This kind of photog- 

 raphy requires a great deal more time to perform than does the 

 multiple-lens photography (pi. 11, fig. 2) used in the preparation 

 of conventional maps. 



Examples of vertical and oblique aerial photographs of Wash- 

 ington, D. C, are shown in plate 12. 



The entire State of Massachusetts was photographed to a scale 

 of approximately 1 to 40,000 in 4 days of flying with a five-lens 

 T-3A camera. The preparation of topographic maps from these 

 five-lens pictures probably will not be completed for several years. 



DEVELOPMENT AND TREND OF AERIAL PHOTOGRAPHY 



The Materiel Division at Wright Field, working in conjunction 

 with camera manufacturers in this country, have produced some 



