asked, however, whether the extra cost for the ground control survey of these additional 
points is worthwhile for the improvement of a few millimetres in accuracy. Similar remarks 
are valid also for the use of correction plates in the Autograph. It must be borne in mind 
also that in both cases only the absolute error would be reduced while the relative error 
would remain practically the same. On the other hand it cannot be stressed too strongly that 
for photogrammetric surveys of this high accuracy the utmost attention must be paid to 
the problem of accuracy of the passpoints. In general, it will be necessary to establish a 
local network of fixed points in which the angles are measured and also distances are 
measured by short-range electronic distance-measuring instruments. Finally, the possibility 
should be mentioned of taking photographs during two independent flights perhaps sepa- 
rated in time. The question arises here also whether the extra cost is justified by the 
small increase in accuracy. 
G. Conclusion 
Precision surveys, with an absolute point accuracy of + 3 cm and distance errors of +1 cm 
are quite rare, but as this example shows, are perfectly possible. Numerical-photogrammetric 
methods are today capable of accomplishing such tasks, usually more quickly, more safely 
and more cheaply than the methods of conventional survey. 
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