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It was felt however that insufficient was known of the behaviour of the APR 
under African conditions, and particularly when using it within such close 
limits of tolerance. For this reason a 'belt and braces' policy was considered 
advisable whereby allowance in the flying programme was made for additional 
photo APR strips at intervals of approximately 25 miles cutting across the 
main flight lines and including some co-ordinated and heighted control points 
positioned by previous triangulation and tellurometer observations, and which 
would be premarked on the ground prior to photography. Only in the event of 
the APR information proving unequal to the accuracy required for setting the 
overlaps individually, would resort be made to this supplementary coverage: 
in practice this never proved to be necessary except as a cross check on the 
work. 
Early in August 1959 a proposal on the above lines was presented to the 
Consulting Engineers, Messrs. Balfour Beatty & Co.Ltd., together with 
detailed estimates of cost and delivery, with and without APR. Although no 
absolute accuracy could be guaranteed since the method had not previously 
been tried, nevertheless it was confidently believed that, provided every 
precaution was taken at each stage of the work, the error in any contour on the 
final map could be limited to + 10ft.. By adopting this method it was estimated 
that mapping of the whole reservoir area (then believed to be about 1,500 square 
miles) could be completed by February 1960 at a cost appreciably below that 
involved if the necessary ground control was to be supplied for mapping by 
conventional photogrammetric methods. 
It says much for the enterprise of the Consulting Engineers, that despite the paucity 
of practical experience in using APR for work of this degree of accuracy, they decided 
to proceed with the operation on the recommended lines, Early in August 1959 instruc- 
tions were given to Hunting Surveys Limited for the flying and groundwork to be put in 
hand immediately the weather conditions permitted. 
FLYING OPERATIONS 
The aircraft used for the main coverage was a D.C.3 fitted with two-stage 
Pratt & Whitney engines and specially modified to carry both the Mark V APR* 
and Marconi AD2000 Doppler equipment in addition to a wide-angle survey 
  
camera. In this case the Doppler navigator was not used for reasons to be 
* Manufactured by Canadian Applied Research Limited.