176 
material will cause a uniform distribution of the errors on the control points of the 
polygon in the different tests obtained by different measurations but by the same pho 
tographs. 
Test 1 Y 
Test 8 X Test 8 Y 
Fig. 32 
It is evident that particularly the slow variations of the errors, namely their 
wide oscillations and not the local perturbations, will be subject to these effects. 
For this reason, the systematic nature of the distribution will appear particularly on 
the distribution of the means M, free from local perturbations.