together with any desired combination of the output recording 
devices* 
Th*' use of the system in conjunction with different photo 
grammetrie instruments impose certain requirements on its most 
important component, namely, the encoder* 
Photogrammetrie work on precision stereocomparators needs 
a discrete value of conversion equal to 1-2 umj for stereocoiu- 
parators belonging to the 2 nd class of accuracy and mechanical 
universal photogrammetrie instruments, this value amounts to 
4-5 nm, and for devices picking-up data from maps, to 10 urn* 
A higher resolution requirement is in direct connection with 
the accuracy of conversion of an analog value tc digital code. 
At present, photogrammetrie instruments developed in the So 
viet Union use two types of encoders: linear displacement-code 
and angular rotation-code. The encoders of the first type give 
more accuracy, since physical principles they rely on allow 
for direct conversion of a linear displacement into a code. The 
encouers of the second type are coupled with load screws of 
photogrammetrie instruments. The intermediate transformation 
of a linear quantity into an angular one affects measurement 
results, since surplus errors, both systematic and random, are 
introduced. However, with accurately made lead sccews, suen 
features of the angle-code encoder as simple'and reliable de 
sign allow for its use even in precision stereocomparatcrs. 
A diffraction type linear displacement encoder used in 
stereocomparator CKB-1 is an example of a lineur-to-coae encod 
er applicable to px^ecision photograrmetrie instruments. 
A distinctive feature of this encoder, as compared with 
a well known diffraction type Fexxranti encoder, is the use of 
two gratings, transparent and reflecting one. This ensures 
better characteristics of the encoder and provides for its 
higher performance. For instance, with the value of a moire 
fringe equal to 4 urn and a gap between tne gratings - of 20 mm, 
the fringe contrast amounts to 0.7* In addition, the encoder 
is not affected by the size of the luminous element and adjust 
ment. errors. An optical schematic of the encoder of stereocom 
parator is shown in Fig. 1. Reflecting difraction grating 1