154 MAPS AND THEIR MAKERS 



treated as plane table stations and rays drawn from them to 

 prominent features portrayed. Details obtained thus are tied 

 in to a relatively small number of points fixed by triangulation 

 on the ground. 



For maps on the larger scales required, for example, for 

 civil engineering projects, much more precise results are 

 obtained from overlapping stereoscopic pairs of photographs in 

 a plotting machine. This operation depends on complex 

 optical principles, but stated very simply it involves placing the 

 photographs in their exact relationship to each other and to 

 the ground surface (thus eliminating tilt). The operator, viewing 

 them stereoscopically, and thus having before him a three- 

 dimensional representation of the surface, is enabled to trace its 

 features, including contours, by the intricate mechanism of the 

 plotting machine. 



To reduce the number of ground control points required, 

 radar navigational aids can be employed to fix the position of 

 each photographic exposure with sufficient accuracy. Professor 

 Hart quotes instances in which areas inaccessible on the ground 

 have been mapped on scales as large as 1 : 50,000 from control 

 stations 250 miles distant. In such cases, however, difficulties 

 in contouring may arise from lack of data. To fill in the detail 

 requires considerable practice in the interpretation of the 

 photographs. Types of soils, rock formation, and vegetation, 

 for example, will reveal themselves to the practised eye. Their 

 appearance naturally will change under varying conditions of 

 light, and, indeed, for certain purposes, the photographs 

 must be taken at a certain time of day or season of the year. 

 The methods and standards of air survey, and hence the 

 expenditure involved, can within limits be varied according to 

 the accuracy required. 



The 1939-45 war gave a great impetus to air survey. 

 Under the direction of the U.S. Aeronautical Chart Service, 

 for example, some 15,000,000 square miles, equivalent to 

 more than a quarter of the land surface of the earth, were 

 photographed from the air by trimetrogon cameras (multiple 

 lens cameras) for small-scale mapping. It has been said that 

 "In the field the aerial camera achieved its final triumph 

 over the plane table as the cartographic surveyor's primary 



