SURVEYS AND MAPS 55 



work proved to be cheaper in a country which would appear to be 

 much more suitable for the camera than is most of Africa. Com- 

 parison of cost is not really very helpful, however, because air 

 survey is valuable particularly in those places where ground sur- 

 vey, owing to lack of communications, can either not be done at 

 all or is unduly expensive. In swampy country such as the sudd 

 region of the White Nile, and possibly round Lakes Bangweulu 

 and Mweru, the advantage lies entirely with air work. In heavily 

 forested land also, the general configuration can be shown more 

 cheaply by air survey, although details like bush paths may be 

 obscured. Such features can best be inserted by ground traverses 

 after the photographic mosaic is complete. 



A topographical map must of course be reasonably accurate in 

 three dimensions, and although contouring from air photographs 

 gives promise of a great future, it cannot yet compete in cost or 

 accuracy with ground work, except in special types of country 

 such as precipitous or broken land covered with vegetation, where 

 ground survey is exceedingly difficult. H. Hemming (1933) gives 

 an example of such a case in South America where air survey for 

 a proposed railway line proved to cost one-eighth as much as 

 estimates for work on the ground. 



A full account of methods used in surveying from air photo- 

 graphs has been published by Major Hotine (1931)- Two methods 

 are commonly employed. One consists of taking oblique photo- 

 graphs and scaling them to the form of a plan by means of a 

 vertical grid. This method shows general topographical features, 

 but is of little value for contours or for ascertaining land poten- 

 tialities.^ The other method employed for contouring from the 

 air depends on taking vertical photographs always in overlapping 

 series, from which trained draughtsmen can plot heights by stereo- 

 graphic methods. The mosaic of photographs together with the 

 map drawn from them gives a complete picture of the land and 

 is most useful for revealing natural resources, but the method is 

 expensive and can only be employed where the land is already 

 known to have high potential value, for mineral resources, forest 

 exploitation or farming. The time required for the cartographical 



* Contoured maps have been made from oblique air photographs for some parts 

 of the world, for instance East Greenland. The necessary apparatus is very costly and 

 does not exist in either Great Britain or Africa. 



