African continent, and, more important, the first occasion when APR has been 
used as control for the contouring of a large area within such close limits of 
tolerance. It may therefore be of interest to sum up in general terms some 
conclusions reached from the results of the survey. 
The most obvious, and the most important lesson, which has been learned; is 
the rapid fall-off in the accuracy of APR with altitude. In theory the tolerance 
for APR circuitry has been specified as 0.01% (i.e. + 1ft. per 10, 000ft. of 
flying height.)* Because this figure is so small there has been a general belief 
that, provided the height chosen is free from local atmospheric disturbance, 
APR accuracy is virtually independent of flying height. While this may be true 
of instrumental accuracy, there nevertheless exist very considerable errors in 
the utilisation of APR which are most certainly dependent on flying height, and 
which may be summarised as follows:- 
1) Misalignment of the radar axis with the optical axis of the 
positioning camera. On level or evenly sloping ground the height 
error caused by this will be directly proportional to the flying height. 
2) Tilt, The effect of this has already been stated in the paper and will 
be directly proportional to the flying height on level ground or even 
slopes. 
3) Spread of the APR beam. The height range of the terrain covered by 
  
the beam is proportional to the flying height if the ground slopes 
  
evenly. 
4)  Side-lobe effect. The diversity of reflecting surfaces encountered by 
the beam will vary with beam spread and therefore with the flying i 
height. It follows that the greater the flying height the more likely 
is 'side-lobe' effect to take place. 
5) Identification, The greater the flying height the smaller will be the 
scale of the 35mm photography and the more difficult will be the 
identification of the profile on the ground. 
From these considerations it is clear that the accuracy of APR control is 
very much dependent on flying height; it will also vary with barometric conditions, 
terrain, vegetation, and quality of flying. Assuming entirely stable conditions, 
such as those occurring on the Niger Survey, one can attribute a 'C"-factor to 
APR somewhere in the region of 500, although this may be rather too high for 
*The Radar Profile and its Application to Photogrammetric Mapping. 
T.J.Blachut & R.D.Leach - Div.of Physics N. R. C.Canada. 
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