terrain clearance is measured by radar or by more precise laser e ^ 
method, the accuracy of the total APR system would not exceed the 
accuracy of the height corrector. Therefore, a significant im- 
provement in the accuracy of terrain elevations obtained by laser 
APR, as compared with a radar APR, cannot be expected. The accur- 
acy of scale determined from terrain clearances is a function of 
the precision with which this clearance is measured. This accur- 
acy is dependent not only upon the techniques employed in measure- 
ment, i.e. radar or loser, but to a larger extent is influenced 
by the quality of the reflecting surfaces, i.e. type of terrain, 
and partly by installation in the aircraft. In summary, the 
accuracy of the total APR system is influenced predominantly by 
three factors: height corrector, technique for measurement of 
terrain clearance and the type of terrain (quality of reflecting 
surface). e e 
A 
The horizon camera is capable of determining the relative 
  
tip and tilt of the aerial camera at the moment of exposure with 
an accuracy of + 2898). It is the simplest, and least expensive 
method of determining and CO with an accuracy which can be 
achieved or surpassed only by very expensive and sophisticated 
guidance systems employed in the space programme. Unfortunately, 
the horizon camera has an obvious shortcoming: namely, it is 
dependent on atmospheric conditions. Nevertheless, under reason- 
able weather conditions it can provide useful data. The author 
has employed successfully the horizon camera on surveys in Nigeria, 
Canada and recently in Ghana. 
Q. QU c FN N 
Ib. 
FC cc c 
Im