INDIA 6 
The accuracy of photogrammetric survey is very much dependent on the accuracy of the ground 
control points and their correct identification on the aerial photographs. The photogrammetric 
survey requires a different type of control points than for the ground survey. The plan control for 
photogrammetric purposes has to be sharp, well-defined and small in dimensions. For higher pre 
cision, it is necessary to signalize the control points before the aerial photography is flown. The 
height control points for photogrammetric survey should be on a level stretch of ground or on a 
very gentle and regular slope. It is, therefore, desirable that photogrammetrically trained persons 
who are fully aware of the photogrammetric requirements, should be employed for the field work 
for the provision of ground control points. It is also desirable that photogrammetric operators have 
field experience as it improves the efficiency of photogrammetric plotting. 
The precision of both the Wild Autographs A7 and A8 is very high and a large enlargement 
from machine to plotting table is possible. In Wild A7, an enlargement of up to 8 times has been 
found satisfactory while in Wild A8, an enlargement of up to 4 times can be safely achieved. Subject 
to fulfilling the requirements of plan and height accuracy, the scale of photography can be l/8th the 
scale of the map for plotting in Wild A7 and l/4th the scale of the map for plotting in Wild A8. The 
precision of Zeiss Stereotope is less and it is generally suitable for small scale topographical survey. 
The enlargement ratio is even less and it is advisable to keep the photo to plotting scale ratio as 1 : 1. 
In marginal cases of small scales of 1 : 50,000 and smaller, the enlargement ratio may be kept at about 
1 • 25 times but should not exceed 1 • 5 times. This enlargement ratio determines the scale of photo 
graphy for survey on Stereotopes. 
The ground testings of photogrammetric surveys in the initial stages have established the 
accuracy of the method beyond all doubt. The process has now been reversed and the photogram 
metric survey methods are being used to check the ground surveys of inaccessible and difficult areas. 
3. Photogrammetric Mapping.—During the years 1960-64, the photogrammetric mapping of 
a large variety of projects on different scales and contour intervals has been completed in the 
photogrammetric units of the Survey of India. Some of the jobs have been a race against time while 
some others had very exacting requirements of precision. The variety was so large that for the first 
five years each job was of a new type and needed very careful planning and execution. New 
difficulties were encountered and solved every time, each of which added to our experience. We have 
now reached a stage where we can plan and execute with confidence any photogrammetric survey 
ranging from 1 : 500 to 1 : 50,000 with appropriate contour intervals provided good quality photo 
graphs are made available. 
Our experience on small contour intervals has led us to conclude that contour intervals of 
2 metres or 5 feet should be the smallest contour interval which should be tried by photogrammetric 
method under Indian working conditions. For smaller contour intervals like 1 metre, 2 feet or 1 foot, 
direct or indirect contouring based on spirit-levelled spot heights tends to be more economical specially 
when the scale of survey permits of smaller scale photography once the restriction of contouring is 
lifted. 
One of the most interesting work in the recent past is the Jarangdih Coalfield Survey on the 
scales of 1 : 1,000, 1 : 2,000 and 1 : 10,000. We were handicapped by the uneconomical design of 
aerial photography which was carried out before the task was allotted to us and the danger of imminent 
monsoon rains in the area of work which restricted the field work time. The maps were required 
within 10 weeks. With a very careful planning from the beginning and the use of I.T.C. - Jerie 
analogue computer for block adjustment of planimetry, the photogrammetric mapping was completed 
in record time to the entire satisfaction of all concerned. 
During this period we also participated in the International Controlled Experiment Reichen- 
bach 1962-64 of Commission IV, Working Group No. 1 of the International Society of Photo- 
grammetry.