: E 
Terrestrial Stereosurveying is used for topographic- 
al mapping of areas of limited extent in regions of 
accidental relief, foot-hills and highlands. 
Thematic maps are compiled under application of the 
same methodology as is used for topographical mapping. 
But these maps differ from topographical maps in their 
contents. For instance, on maps of agriculture the agri- 
cultural lands ( arable, longfallow, haying ) with de- 
tailed characteristic of each particular area etc. are 
shown. On maps intended for land-reclamation projects, 
most detail is given concerning existing and old drainage 
and irrigation systems, underground communications and 
other elements that are important for solving special 
planning problems. 
Besides that, relief on such maps is generally re- 
presented with smaller contour-interval, the micro-relief 0 e 
is marked out, and the number of spot-heights is increas- ] 
ed. On irrigated lands these points are located at inter- \ 
sections of the lines of a square grid with sides 20-40 m. 
Of higher informational value are the photomaps. 
These enable to improve quality and efficiency of pros- 
pecting work carried out in connection with soil recla- 
mation, cultural and technical projects, hydrogeological 
and other reconnaissance. Within most densely populated 
regions, the necessity of map revision, as regards situa- 
tion, arises in 6-10 years after their compilation; 
within less populated areas - after 10-15 and more years. 
Map revision is most effectively executed by means 
of a new aerial survey. In such cases the aerial pictures 
undergo office interpretation. If only a few changes 
have taken place - the map receives only the necessary 
corrections. If, instead, essential changes have been 
discovered, the map as a whole,or some parts of it are é © 
compiled anew. 
Revised maps undergo complete or partial check on 
the spot. Identification is most effectively done from 
a helicopter,