the planimetric accuracy of orthophotos. The Stereocompiler measurements 
were, however, used as supporting results and for the elimination of 
possible measuring blunders, 
Planimetric Accuracy of Orthophotos 
The orthophoto coordinates of the test points, measured on the 
Monocomparator and the Stereocompiler, were transformed to the correspond- 
ing ground control values determined on the A7 by a similarity transfor- 
mation using a least squares fitonall measured points. Standard errors 
were computed from the residuals. 
The respective results from Monocomparator and the Stereocompiler 
measurements indicated the same general error pattern. Due to the lower 
measuring accuracy of the Stereocompiler, the standard errors computed 
from the Stereocompiler measurements were slightly larger than the 
corresponding values obtained from the Monocomparator measurements. The 
independent sets of measurements from both instruments were used to 
check the individual coordinate measurements and to reject those occasional 
points at which gross identification or measuring errors were committed. 
The stereoscopic measurements generally indicated a satisfactory 
agreement with the results obtained fromthe Monocomparator measurements. 
The root mean square values of the X-parallaxes, measured on the Stereo- 
compiler from the 1:2 500 orthophoto pairs, are listed in Table II. 
These values are the combined result of X-errors in both orthophotos 
submitted for each instrument. Due to variations in the coverage of the 
different orthophotos, the total number of control points varied for 
each orthophoto. 
It should be noted that the main sources of errors, such as 
inaccurate profiling or excessive slit size, introduce radial errors in 
the position of orthophoto points. Other sources, such as film deforma- 
tion or limited measuring accuracy, may have a radial as well as a 
tangential effect. In order to differentiate between these two groups 
the standard errors in planimetry were defined by their radial and 
tangential components. The results from the Monocomparator measurements 
are listed in Table III. 
For some orthophotos the number of measured points, indicated 
in Table III, exceeds the maximum number of control points (200). This 
is because the measurements of some orthophotos were repeated as double 
checking; the values listed represent the combined result of these 
measurements.