SURVEYS AND MAPS 59 



Bahr-el-Jebel.* The work was hampered by bad visibihty, par- 

 ticularly in the dry season, and the country is of course difficult, 

 but it is quite certain that the mass of detail about the swamp areas 

 could have been collected in no other way. For the purpose in 

 view, precise levelling is required, of a higher standard than is 

 usually included in ground control for air survey. The main lines 

 of levels are already complete (Hurst and Phillips 1933). 



In French West Africa, over the Gold Coast border where the 

 country is open and the rivers give easy ground control, consider- 

 able areas have been photographed with success, and these activi- 

 ties are being extended. Another foreign area where air survey 

 combined with thorough ground work has proved successful is the 

 Lower Belgian Congo. This was done by the Compagnie Aerienne 

 Frangaise. Apart from this and the survey of the boundary be- 

 tween Katanga and Northern Rhodesia referred to above, no air 

 survey has been attempted yet in the Belgian Congo except in a 

 small corner of Katanga, where some air photographs have been 

 used in preparing the detailed series of pubHshed maps. It was 

 found to be too expensive and less efficient in results than ground 

 work and so was discontinued, but this small experiment cannot of 

 course be held to show that air survey is of no value in Katanga. 



In the case of the surveys carried out by commercial firms under 

 special contract for mining companies, the results are confidential 

 and are not released to science for several years. Where photo- 

 graphs have been made available for examination, little advantage 

 has so far been taken of the opportunity. More extensive use could 

 be made of these results if local survey officers were trained to 

 interpret them, but it must be remembered that a scientific under- 

 standing of the geology, soils, and vegetation of the region in 

 question is at least as helpful in the interpretation of aerial photo- 

 graphs as any special training. In those parts of Africa where 

 organized air surveys would provide the best basis for develop- 

 ment the question of cost is important. The high cost of the air 



' The proposal has been to construct an artificial watercourse from Mongalla, to 

 the east of the sudd area, in order to join the existing channel of the River Sobat, 

 which enters the main White Nile at Malakal. The result would be to circuit the 

 sudd area, thereby reducing the enormous wastage of water which the sudd entails. 

 Probably such a flow will be left along the present channel of the Bahr-el-Jebel, as can 

 be carried without loss of water. This will enable water transport to follow its present 

 route. 



