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-5- 
The standard deviation of the means of groups of reliable points covering 
two models has been established in the above mentioned investigation with 
the following results: 
High altitude APR: m 0 s.e. of groups of 9 reliable points: 1.4 m 
Low altitude APR: m Q s.e. of groups of -|8 reliable points: 1.1 m 
With respect to the Laser APR, no definite evaluations are yet available. 
Considering the extreme accuracy of the laser radar component, it can be 
estimated that the total accuracy will depend mainly on the hypsometer 
component. An accuracy of 1-2 m standard deviation can thus be expected, 
depending to a certain extent on the flying height, but much less influenced 
by the topographic characteristic of the terrain than in the case of the 
conventional APR. 
Geometrical Meaning of APR data in photogrammetrie system» 
The combined information of hypsometer and radar altimeter results in 
absolute terrain heights relative to the isobaric reference surface. 
After the application of the Henry correction and allowing for an additional 
constant height correction (index error) and if necessary an additional 
linear correction to allow for residual slope errors of the isobaric 
reference surface, this data can be directly used as ground height control. 
If the two components are considered separately, the hypsometer data 
correspond to the bz-values of the exposure stations (see foregoing 
paragraph), while the radar altimeter data, as registered at the exposure 
stations, will correspond to the difference (Z (principal point) - bz.). 
Thereby an absolute distance is defined, resulting in an independent control 
of the scale of each model. 
This results in two possible ways of applying the APR data to aerial 
triangulation. 
A) Use of the combined data to replace ground control,. 
This alternative is only to be preferred in cases of rather flat areas, 
there being a rather large number of well defined APR points available. 
B) Use of hypsometer and Radar-altimeter data separately. 
This method will lead to better results in cases of considerable 
elevation differences in the terrain, where only a limited number of 
reliable APR points are available, as it allows a separate control of 
the strip deformation due both to accumulated 9-errors and scale 
transfer errors. The terrain conditions can impair the reliability of 
the scale control, whereas they have no influence on the hypsometer data 
and so reliable information can be obtained at all exposure stations.