29 
y 
x-scale 
Photograph 
l 
mark 
FIG. 21. 
Principle of the measuring system. 
their origins correspond with the location of the measuring mark. During tracking, the scales 
together with the measuring mark are displaced with respect to the xy-axes and the photograph. 
These displacements are measured automatically in intersection points between the scales and 
axes. The system is not very sensitive to small imperfections in the tracking device (e. g. 
differential rotations) and temperature changes. The accuracy depends primarily on the x and 
y reference lines and on the precision of the linear x,y scales. 
The observation system is provided with binocular zoom optics facilitating optical magnifi 
cations between x2 and x60. 
Measurement and recording is realised automatically. For this purpose, specially designed 
optical-electronic digitisers for coarse and fine readings are used. The fine readings are 
obtained from optical micrometers operated by means of photo-electronic elements. 
A control panel, control unit with a storage and the conventional recording units are provi 
ded. 
V. 3 Point marking instrument Kern PMG1. 
The PMG1 is provided with three stages for simultaneous marking of selected identical points 
on three overlapping photographs (fig. 22).