4 
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PHOTOGRAMMETRIC ENGINEERING 
changeable cam disc. The profiles are 
scanned in the F-direction at a speed regu 
lated by the operator, the instrument auto 
matically indexes in X, but the Z-corrections 
are introduced by the operator with the 
right-hand wheel and transmitted to the tape 
punch through the impulse generator. 
The orthoprojector, which projects the 
imagery, is a photocarrier of the same type 
as used for model scanning in the C8. This 
unit is shown in Figure 2. The complete 
analogy between the scanner and the projec 
tor permits optimum imagery as well as 
transfer of setting values. Moreover, the in 
strument can be oriented to preplotted con 
trol points, so that rectification or a check of 
orientation can be made independently of 
the readout desk. 
New Methods of Differential 
Rectification 
The two methods of differential rectifica 
tion mentioned above can only be employed 
with specific plotting equipment. They yield 
exact results only when operating with con 
gruent bundles of rays. Thus, for every new 
focal-length a special projector must be used. 
Drobyshev proposed using the Stereograph 
SD with auxiliary equipment for differential 
rectification. The affine transformation would 
avoid dependence on a particular focal- 
length. Nonetheless, even in this proposal 
differential rectification is possible only in 
Fig. 2. The Gigas-Zeiss Orthoprojector.