he 
W 
ued 
TT 
uired 
ts 
re, 
ition 
.ng 
made 
reas 
At 1:1250 the work consists of upgrading the scale of survey in developing urban 
areas. In the period of this report over 350 km? 
have been mapped by precise 
stereoplotting machines, using normal angle (305 mm focal length) photography 
at 1:5000.scale. 
In rural areas the old 1:2500 County Series maps have continued to be recast on 
the National Grid and revised where they are suitable (117). During the period 
about 37 000 km^ have been revised by graphical air survey using some 59 000 
rectified enlargements at approximately map scale. In about 900 km^ , where 
dense revision was required or where the terrain was unsuitable for graphical 
methods, stereoplotters provided a more economical solution. Normal angle 
photography at 1:7500 scale was used. As the programme nears completion the 
load is decreasing and only about 8000 rectified enlargements are now being 
produced each year. Routine tests of accuracy of these maps are performed 
using points of detail co-ordinated by analytical aerial triangulation on small 
scale photography. Where suitable County Series maps are not available, rural 
areas are being resurveyed at 1:2500 scale using precise stereoplotters with 
normal or wide angle (152 mm focal length) photography at 1:7500 or 1:10 000 
scale respectively. Some 1500 km^ were resurveyed during the period. 
Mountain and moorland areas are being resurveyed at 1:10 000 scale by stereo- 
plotters using 1:25 000 scale wide angle photography. During the four years this 
work spread from Scotland into northern England and to Wales; detail and contours 
for about 25 000 km? were surveyed. The 1:10 000 series is the largest scale 
at which contours are shown. Where the basic scale of survey is larger, contours 
are added by stereoplotting machines to 1:10 000 maps derived from the larger 
scales of survey. Metric contours have been provided for some 36 000 km? of 
derived mapping during the period, including some recontouring for areas 
previously contoured with 25 ft vertical interval. It is expected that metric 
contours will be complete for the whole country by about 1985. The metric 
contour interval is 5 m, increased to 10 m in mountainous areas. 1:25 000 scale 
wide angle photography is generally used but in the flatter areas of eastern 
England super-wide angle (88 mm focal length) photography at 1:31 000 scale is 
preferred. 
An increasing amount of continuous revision is also being carried out by air 
survey where developments are large enough to justify it. Between 1972 and 1975 
some 4500 km’ were revised by air survey methods. 
In addition to its work on the national survey, the Ordnance Survey makes a 
small part of its resources available to other organisations and to the public on 
repayment. Apart from the supply of prints, diapositives or enlargements of 
air photographs, existing photography can be used to supply non-standard 
products, such as contours at 2 m intervals on 1:2500 scale maps, and a number 
of small repayment tasks of this nature have been carried out, mainly for local 
authorities and public utilities. 
The Department carries out all stages of the aerial survey, including flight 
planning, exposing and processing the photography (including infra-red for tidal 
surveys), aerial triangulation where required, instrumental or graphical plotting, 
and field completion. Aircraft and crews are hired from commercial companies 
between March and October each year, but the Ordnance Survey provides its own 
camera operators and cameras. Four or five aircraft flying an average total of 
800 hours per year have been employed from 1972 to 1975. During this period 
the variable British weather has provided both the best and the worst air photo- 
graphic seasons on record, with survey areas photographed annually ranging 
from under 16 000 km^ to over 56 000 km? . A small amount of photography has 
also been taken in winter on an opportunity basis.