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13. 
formula for variances in terms of ray inclination. These results for 
the super-wide angle camera show significantly the same variances as 
for the wide angle camera, except for low contrasts where the preci- 
sions deteriorate rapidly. 
For high flying heights (Table 2) the camera lens is the major 
factor affecting image quality, but the increase in variances in terms 
of incident ray inclination is large only for the high optical magnifi- 
cations, where for the lower quality camera 188 the variances deterio- 
rate by a factor of 3.7. 
Overall, there is little difference between the high and low 
flying height cases for each camera individually, but there are 
large differences between each camera. Camera 189 is overall the best. 
For the lower quality camera 188, variances for high ray inclinations 
are larger for 20X than for 10X magnification and there is therefore 
no benefit gained from using the high magnification when viewing 
photographs from that camera. On the other hand for the high quality 
camera, an improvement in variance of about 2.4 times is achieved by 
using the higher magnification. 
2.8 Conclusions 
(i) The details of the study summarized in Tables 1 and 2 
demonstrate variance ratios of x and y-coordinate observations as a 
function of image quality of the target and observer capabilities. 
Variances given do not include any contribution of stereocomparator 
precision which must be added to those in Tables 1 and 2. The inter- 
related factors of image quality, optical magnification and target 
contrast must be combined to obtain a true picture of their effects; 
target contrast however is the major factor affecting variances. 
(ii) The highest precisions obtained of the order of (2.6um)? 
at an optical magnification of 20X, are very much larger than the 
highest precisions obtained by O'Connor (1967), (l1 sec of arc is 
theoretically equivalent to 0.064m at 20X magnification). However, 
the precisions which can be obtained clearly approach the highest 
precisions of todays' photogrammetric equipment. On the other hand, 
there is clearly the possibility of even higher precisions if image 
quality can be improved. 
(iii) The only assumptions made in this study are that the shape 
of the combined MIF of the system approximates a Gaussian function, 
and that granularity can be included in the manner proposed. As there 
is justification for both these assumptions it is felt that the claims 
made in this paper are justified, and applicable to photogrammetric 
practice. 
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