54 SCIENCE IN AFRICA 



in relation to other branches of development, it seems best to con- 

 sider the value of air photography in two aspects: (i) as a method 

 of making maps, and (2) as a means of showing the resources of a 

 country apart from the mere plotting of physiographic features. 



It should be realized that the air photograph cannot in any way 

 replace triangulation, the fixing of a number of prominent features 

 to form the groundwork for a map. It is in the later stages of sur- 

 vey, for which the plane-table is generally used, that the camera 

 has many advantages. For topographical air survey in country like 

 so much of Africa, where the triangulation network is incomplete 

 or non-existent, strong ground control parties are required in order 

 to fix the position of features which can be recognized in the 

 photographs. The minimum distance between ground control 

 points depends, of course, on the scale of mapping, but as a rule 

 such points cannot be more than about ten miles distant from each 

 other on account of photographic distortion. Using a wide-angle 

 lens or a multilens camera for small-scale work, it is claimed that 

 the distance can considerably exceed this figure without loss of 

 accuracy. 



In highly mapped countries like Great Britain air survey has 

 proved of much value in bringing existing maps up to date by 

 showing recent developments in the form of buildings, roads, and 

 rails, and this type of air survey has been applied also in Tangan- 

 yika in connection with town-planning, etc. It represents a special 

 case, however, and has little to do with the question at issue — ■ 

 whether the aeroplane and camera have advantages over the theo- 

 dolite, level, and plane-table in the mapping of unsurveyed areas. 

 In putting forward a few arguments both for and against air sur- 

 veying in the following paragraphs, it is not intended to suggest 

 that it is an alternative to ground surveying, but rather that the 

 aeroplane and camera are instruments of survey which can with 

 advantage be added to those in more general use. 



The time and cost of air and ground surveys have often been 

 compared. At a discussion on Topographic Air Survey at the 193 1 

 Conference of Empire Survey Officers (1932), it was concluded 

 that for small areas of open undulating country the plane-table is 

 cheaper, but for large areas this is probably not the case. Against 

 this, recent experience of the Survey of India was that ground 



