general use, When the APR factor is compared with the usual photographic 
C-factor of say around 1,000, it is seen that the APR record cannot normally 
give the required accuracy unless it is flown from half the altitude at which 
it is economic to carry out photography. APR flown simultaneously with 
photography will therefore only be practicable in circumstances when the 
vertical tolerance in the map contours is such that the flying height deduced 
from the APR "C"-factor coincides with that of the service ceiling of the aircraft. 
Fortunately the generalisation which has sometimes been made that "only if 
APR and survey photography are flown simultaneously can APR be economic" 
is not borne out by the results of the Niger Survey, nor is there any reason to 
believe that this will be so in other areas for the following reasons ;- 
1) The additional cost of an aircraft unit fitted with APR and including 
the specialist to operate it, will not normally exceed 50% of the 
standing charge and say 5% of the hourly flying charge. 
2) Since APR can be flown at any height down to 4, 000ft., and the 
lower the better (subject only to absence of turbulence), and since 
small amounts of cloud can be tolerated on the 35mm photography 
(i.e. here and there a principal point may be lost), it follows that 
APR flying can often be carried out on dayswhich are unsuitable for 
survey photography, when the aircraft would otherwise remain idle. 
A further economy can be introduced if the electronics engineer is 
prepared to serve a8 camera operator on days when the survey 
photography is being flown. 
3)  Byflying the APR control along the lateral overlaps of the survey 
cover instead of down the centre of the runs, it has been shown that 
the requirements for aerotriangulation (other than for purposes of 
scaling and for tie strips) can be eliminated, thereby effecting a 
major economy both in time and expense during the photogrammetric 
stage. (The 'smoothing out! process whereby false APR readings 
are eliminated, takes place first during the analysis and reduction of 
the APR strips and secondly in the settings of the individual overlaps). 
The results of the Niger Survey have demonstrated that, (with APR control 
flown at the correct height along the lateral overlap) it is possible to span 
distances of 60 - 80 miles without intermediate ground control and still give 
no final height error on the map greater than 10ft. The conditions under 
which this survey was undertaken were certainly not exceptional and there 
Seems no reason to doubt that they were anything but average. If this fact be 
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