— Absolute mean square point error: + 2,1 cm 
— Mean square distance error: + 0,6 cm from check lengths 
+ 1,2 cm from standard switch dimensions 
— Maximum distance error: 2,2 cm from check lengths 
— Gross errors in photogrammetry: 2,7 cm from standard switch dimensions 
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The mean square point error of approximately + 2 cm corresponds to an error of ap- 
proximately 7 um in the negative or approximately 0,05 9/oo of the flying height 
above ground. It must be borne in mind in considering this figure that the distortion 
of the camera lens, of maximum 0,01 mm, was not eliminated by correction plates 
and therefore is present in the total error. The real accuracy of setting may be taken 
as approximately 3 um in the negative, as is made clear in the following treatment: 
The ground marks of 10 cm x 10 cm square were imaged in the negative as a square 
of approximately 35 um in which the measuring mark, with a diameter of 20 um, 
was placed. As long as the measuring mark was placed within the signalized mark the 
maximum observation error would be 7,5 um; it may be assumed that the mean 
square setting accuracy would be approximately half this maximum error. Expressed 
in model coordinates the mean square point error corresponds to approximately * 0,02 mm 
and the mean square error of distance between neighbouring points + 0,01 mm. 
In Finland accuracies of m= m_ =+2.6 cm have been achieved in photogrammetric city 
surveys, corresponding to a total mean square position error of approximately + 3.7 cm. 
The scale of photography was similarly approximately 1:3000; the plotting was done in a 
Wild A8 Autograph. This example also shows that with the help of modern equipment and, 
computer programs numerical-photogrammetric surveys with an accuracy of a few centi- 
metres are entirely possible. 
The high accuracy of the Muttenz survey, until now scarcely expected even in specialist 
quarters, is attributable to a perfectly adjusted Autograph, experienced operators and a very 
efficient coordinate transformation and adjustment program. Another reason for the good 
results is the fact that the photographs were taken on an overcast day so that the effect of 
irradiation of the signalized points was avoided. It may be noted that surveys with such a 
high specification of accuracy must be flown at low heights so that the aircraft will almost 
always be below any layer of stratus or alto-stratus cloud. 
A further increase in the precision is possible if, instead of single passpoints, groups of 2 or 
3 passpoints are used; in this way the accidental observation errors in the passpoints, which 
have a systematic influence on all other points, could be reduced. The question must be 
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