INDIA 3 
the contour interval. So far we have very seldom used stadia, or barometric 
height traverses, or astronomical fixes for small scale mapping. The main 
reason is that the country has a well established network of first order tri 
angulation and levelling and no difficulty is ever felt in breaking this down to 
cover any additional areas as and when required. The co-ordinates of control 
points throughout India are either in spherical terms or converted in terms 
of the Lambert Conical Orthomorphic Grid. 
With the advent of photogrammetry, it is hoped that the amount of 
ground control required for mapping will be reduced considerably and as new 
developments in photographic and plotting techniques take place, the density 
of control will be further reduced. 
At present when using aerial triangulation adjustment is eifected 
graphically by drawing correction curves separately for x, y and z for the 
3 sections of models ( upper, middle and lower ). The density of ground con 
trol generally stipulated for a strip is 5, 3 and 5 points for the starting, middle 
and end models. In addition we are carrying out experiments with tie runs, 
which if successful, will further reduce the above mentioned control. 
Mapping.—The equipment mentioned earlier in this report is intended 
to be used for mapping in India as outlined below : — 
Wild Autograph, A.7.—For aerial triangulation and large scale surveys. 
Although not economical, this machine was recently used, mainly for training 
purposes, for the mapping of a densely populated area on the 1 : 15,000 scale 
with contours at a vertical interval of 20 ft., the ultimate map being pub 
lished on the scale of 4 inches to one mile. Photography was carried out at 
the 1:40,000 scale with the Wild R.C. 5a camera using the 11-5 cms. 
Aviogon lens and flying at a height of about 15,000 ft. above the general 
level of the terrain. Ground control provided by triangulation was supple 
mented by aerial triangulation. 
Wild, A.8 Stereo-plotting Machines.—For large and medium scale surveys. 
W illiamson-Ross Multiplex Equipment.—Mainly for 1 : 25,000 scale 
surveys. 
Zeiss Stereotopes.—Mainly for surveys on the 1" to 1 mile scale, which 
is our basic scale for surveys in India, using aerial photographs on the scale of 
1 :40,000. 
Zeiss Rectifiers.—For large and medium scale surveys of flat terrain 
where no contouring is involved, and for the preparation of rectified photo 
mosaics.