THE PROBLEM 
By ground levelling the minimum height control required for photogrammetric 
mapping of the reservoir area was estimated to take a party of 10 men four 
months to complete. The rains were unlikely to cease before early September, or 
possibly later, which meant that vertical photography could not be undertaken until 
mid-September at the earliest and the levelling programme might, therefore, 
extend until the end of February which was the latest date for delivery of the maps. 
THE SUGGESTED SOLUTION 
There appeared to be only one possible way of carrying out the work in 
time - by the use of the Airborne Profile Recorder. APR control so far as was 
known,had never previously been employed for controlling block mapping with a 
contour interval of 20ft. accurate to + 10ft. at worst and with the desirability of 
+ 5ft. accuracy if this could be achieved. 
Having decided in principle on the use of APR, consideration was given to 
the possibility of averaging out the residual errors of the APR lines by aerotrian- 
gulation of each photo strip. This however would have entailed an additional six 
weeks of machine work (as well as adding to the cost) and was ruled out by the 
timing of the programme. Instead it was planned to eliminate aerotriangulation 
entirely and to rely on setting each overlap individually on control taken from 
three independent APR profiles; the central APR line would be flown from a 
height of 20, 000ft, a3.m.5s.1. simultaneously with the vertical photography and 
would therefore require no additional flying. Further flights, with APR and 35mm 
recording camera only,would be made along the lateral overlaps of the photo 
cover at heights between 5, 000 and 10, 000ft. (the actual heights of these control 
runs being adjusted to meet prevailing cloud and barometric conditions and to be 
left to the discretion of the aircrew). 
Each APR profile would be 'held! at its ends by tie strips controlled by 
ground levelling and would be further checked against an additional tie strip 
provided approximately half-way between the north and south limits of the 
reservoir. Because the country was in many places covered by thick bush it was 
considered that it would take too long to position the terminal tie strips in the 
ideal position cutting at right-angles across the ends of the main flight lines. 
For this reason the tie strips were planned wherever possible to follow existing 
roads and to make use of the block control to be established at the Kurwasa dam 
Site and at Yelwa Township. The plan was to fly these "road ties" in the period 
July/August below cloud-base during the wet season. By confining the strips