The Use of Radar Altimetry in the Mapping of a Reservoir 
by P. G. Mott 
In July 1959 Hunting Surveys Ltd. were commissioned to produce a 1:50, 000 
scale map with 20 ft, contours to cover a projected reservoir area of about 
1,500 sq. miles on the Niger above Jebba; the urgency of the scheme called for the 
maps to be delivered to the consulting engineers by February 1960. Photography 
of the area was precluded by the rains until September, and levelling for height 
control by normal ground methods was likely to extend the field programme until 
February, the date when the maps were due for delivery. It was decided that the 
only way to produce the maps on time was to base the height control for photogram- 
metric mapping on data supplied by the Airborne Profile Recorder. 
A plan was adopted whereby each strip of photographs was supplied with three 
APR profiles; the central line flown simultaneously with the survey photography at 
20,000 ft., while purely APR lines were flown along the lateral overlap of the 
photo cover at heights between 5,000 and 10,000 ft., to suit prevailing weather. 
The APR pattern was 'held' by tie strips, controlled by ground levelling. 
Subsequent analysis of the APR records showed that the "error spread" from 
the central lines flown at 20, 000 ft. was much greater than that from the lateral 
lines flown below 10,000 ft., and it was therefore decided to discard the high level 
record, and set the overlap in the plotting machines solely from the low level APR 
records in the lateral overlaps of the survey photography. No aerotriangulation was 
required and the work of plotting topographical detail and contours was divided among 
a number of Wild A. 8 plotters. 
The following facts are worth noting - 
1. Ground levelling to control the APR profiles took 38 man-weeks compared with 
the estimated 170 man-weeks need to provide normal height control for 
photogrammetric mapping. 
2. The contoured maps of the whole reservoir area were delivered within three 
months of completion of photography and APR flying. 
3. The cost of the survey with APR control showed a saving of about 23% over the 
estimated cost of the job without APR. 
4, 80% of the 192 APR points checked against ground levelling showed no error 
greater than 5 ft., and 100% were within 10 ft. 
5. The main lesson shown by the survey is that in practice the accuracy of the APR 
falls off rapidly with increasing altitude. 
6. Amajoreconomy is effected, both intime and expense during the photogrammetric 
stage, by flying the APR control along the lateraloverlaps instead of downthe centre 
of the runs, thereby eliminating the requirements for aerotriangulation,