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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.