If self-calibration is used, however, then we need proper tests to 
separate errors in ground control networks from systematic deformations. 
The simple tests used to date /7/, /8/, are not sufficient. More 
elaborate testing procedures are required, such as proposed in /4/ and 
/10/. A proper separation of different sources of block deformations by 
such elaborate testing procedures also implies special requirements for 
block- and network design /13/. In fact, the design of blocks and net- 
works should be completely integrated. 
ACKNOWLEDGEMENTS 
The authors like to thank Ing. E.K. Zwintzscher, Department of 
Photogrametry ITC, for carrying out the necessary computations. 
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