Group 2 theoretically ought to amount to 2.6 times the mean square errors of Group 1. 
But the ratio is only 1.3 instead of 2.6. This smaller ratio indicates that other sources 
of error exist, which are effective independently of the location of the points within 
the profile strips. A part of the errors may be explained as tapping-off errors, influences 
of altimetric errors (see hereinafter), and film shrinkage. 
Mean square errors from errors of altimetric setting 
The photomaps 1 and 2 have been plotted with the same orientation in the Stereo- 
planigraph and in the Grthoprojector, with the same point of origin, and thus with 
equal position of comparable points within the profile strips. They cover the same area 1. 
Out of the differences of coordinates on these two maps the mean square errors can be 
calculated. They only contain the effects of altimetric errors made by the operator in 
profile scanning during the plotting process, and the distortions caused in the treatment 
of the exposed film. The maps 1 and 2 comprise 61 identical points, by means of which 
the mean square coordinate error of a measurement has been calculated. We obtained 
m c = 17 cm in nature, i. e. 0.06 mm on the map (m x = 15 cm and m y = 19 cm). In order 
to determine these values of error, all points have been used regardless of their location 
within the profile strips. These values constitute a measure for the reproduceability of 
the maps on equal plotting conditions. 
From the maps 1 and 4, as well as from 2 and 4, mean square errors can also 
be calculated out of the differences. But in these differences the effects of all sources 
of errors, as mentioned above, are present. Therefore the mean square errors of a single 
measurement ought to be equal to the absolute errors. We obtained 
m x 
m u 
m c 
[cm] 
[cm] 
[cm] 
from map 1 and 4: 
35 
29 
32 
from map 2 and 4: 
38 
30 
34 
as mean value: 
37 
30 
35 
0.11 mm on the map 
The conformity of the mean square coordinate errors within 10% is satisfying, with 
regard to the limited test material. 
In order to compare them with the errors of Orthoprojector maps we also indicate 
the mean square coordinate errors of plottings in the Stereoplanigraph (normal angle 
photographs 1 : 8,000, plotting 1 :2,000, approximately 2,400 points, plotted by nine 
operators). These measurements resulted in mean square coordinate errors amounting 
to an average of 17 cm in nature, i. e. 0.08 to 0.09 mm on the map. 
In conclusion the following errors have been obtained: 
— From the four photomaps tested: 
Absolute mean square coordinate error 
out of all points: 37 cm in nature = 0.12 mm on the map