4 GREAT BRITAIN 
REPORT OF THE ORDNANCE SURVEY 
(Note: These answers apply ONLY to the Ordnance Survey of Great 
Britain and NOT to any other British organisation). 
A) a) No maps on these scales are made by photogrammetric means. 
g) Existing 1:25 000 maps are revised by simple radial line methods. 
Heights are not revised by thıs means. 
B)1 a) A small area (the county of Essex) is being surveyed at 1:2500. 
Certain towns have also been surveyed at 1:1250. The contact scale of 
the photography is in each case about half the final scale of the plans. 
b) Scales as given above. The planimetry only is supplied from 
air photographs. This is fixed from triangulated points about 7 km of 
apart. This control is broken down for the 1:2500 scale, by means 
Cambridge stereo-comparator observations and subsequent computation, 
to produce air observed control about 400 m apart. For the 1:1250 scale, 
control at about the same density is supplied by ground methods. The 
air observed control is found, by testing by ground control methods, to 
provide planimetrie co-ordinates with a standard deviation of about 
0,75 m. 
Detail is fixed, at both scales, by radial line methods. 
Accuracy of detail at 1:2500 is about 1,5 m and at 1:1250 is about 
0,5 m. All topographical detail, which can be shown at the scale, 1s plot- 
ted; except heights. 
The plans are all based on the National Transverse Mercator Pro- 
jection and Grid. 
The plans are « finalled », i.e. given a careful check, on the ground. 
c) Existing 1:25 000 and 1:10 560 maps are revised from air pho- 
tographs by simple methods. Heights are not revised. 
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