o 
— 44. — 
Situation accuracy is characterized by & m.s.e. 
+ 0.5 m, height error = 1/4 - 1/3 of the accepted con- 
tour interval. 
In large-scale mapping ( 1:25 000 - 1; 2 000 ) the 
scale of photography is usually two to three times 
smaller than that of the map. 
The relation between the scales of mapping and 
photography is in each specific case determined in de- 
pendence on the characteristics of the country under sur- 
vey ( outline, relief,etc.),and the proposed method of 
piotting. | ! 
For mapping at 1:25 000 scale, terrain is photo- 
graphed at 1:50 000 - 1:50 000 scales. Here a simultane- 
ous photographing with two cameras is sometimes also 
executed ( in a larger scale - for identification ). 
Relief survey is most generally made with aerial cameras 
of 70 and 100 mm focal length; for making photoplans, 
cameras of a focal distance 200 mm and more are used. 
Populated centres, especially those of multistory 
building, are photographed with cameras of 250 - 500 mm 
focal length. 
The focal distance is in each specific case chosen 
depending also on the character of the surveyed terrain 
and technique of plotting, and more especially - on the 
method of relief surveying and the accepted contour 
interval. 
Longitudinal overlap of photographs is approximate- 
ly 60$, transversal overlap - about 20$. Horizontal and 
vertical bridging is, as a rule, thinned out. The points 
of the main geodetic control are marked before starting 
the survey flight. 
In the USSR, much attention is paid to the problem 
of reducing the volume of field geodetic operations 
connected with establishing horizontal and vertical 
control for aerial surveying;this is of quite particular 
importance when areas of difficult access are mapped.