he 
untry 
® 
- 23 = 
in 1932 when a short time after the introduction of the 
photogrammetric methods in Bulgaria there commenced pro- 
cessing in plotting the topographic map at scale 1:25000. 
When working out the topographic map at scale 
1:25 000 as well as the economic map at scale 1:5 000 
initially there was carried out a field topographic de- 
ciphering, white later laboratory interpretation began 
to be applied more often. 
"hen laboratory interpretation is not suggested, 
field interpretation is performed before the processing 
begins in accordance with universal and differential 
stereogrammetric methods. Only in forest regions with 
dense plants reaching a height over 4 metres when pho- 
tographed in full leaf, stereoplotting is uncertain, 
then preliminary field interpretation is not performed. 
Basic interpretation materials in processing topo- 
graphic maps are contact photoscopy, photoschemes and 
photoplans. 
Up to the present chiefly black and white photo 
images are used. Colour photos are used only in inter- 
preting scientific research work. 
In Urban regions in some cases beside the photo 
schemes, there are made large scale photos of the proe 
cessing map. 
Photoschemes are worked out according to map sheets 
approximately at the scale of the plotted map. 
The approximate scale of the photo schemes is ob- 
tained by means of a phototransformer , while the 
mounting of the individual photos is mounted by means 
of an optic assembly.