INDIA 3 
Table 2 gives the radial triangulation performed in the Survey of India during the years 1960 
to 1964. 
Table 2.—Radial triangulation in India during 1960-64 
Scale of 
combination 
Area in 
sq. km 
Approx. No. 
of photographs 
Remarks 
Survey of India 
Departmental Survey 
1 : 10,000 to 
11,633 
1,248 
Departmental Survey 
1 : 30,000 
Smaller than 1 : 30,000 
3,15,453 
25,796 
Development Project Survey 
1 : 10,000 to 
522 
98 
Development Project Survey 
1 : 30,000 
Smaller than 1 : 30,000 
12,962 
560 
2. Aerial triangulation in space.—The method of aerial triangulation in use in India is the Aero- 
polygon method which the French call “cheminement aerien” or aerial traverse where the airbases 
are considered as traverse lengths. Aerial triangulation is done on Wild Autograph A7 instruments 
with the Zeiss Parallelogram, with alternate bases inwards and outwards. The starting stereopair 
is always provided with sufficient ground control points for absolute orientation in scale and horizon- 
talisation. The successive stereopairs are connected to the first by relative orientation of one ( new ) 
camera and scale transfer. 
When the aerial triangulation was first started in 1955, the relative orientation of a pail of 
photographs was carried out by the empirical optical-mechanical method of von Gruber. The dis 
advantage of the von Gruber method is that consistent relative orientation is not possible due to differ 
ence in personal appreciation and measurement of parallaxes by different operators. This results 
in a systematic error in the relative orientation which is different for the different operators. The 
aerial triangulation of any large area will necessitate employment of a large number of operators and, 
therefore, the propagation of errors in a strip of aerial triangulation, different stereopairs of which have 
been relatively oriented by different operators, will be of a heterogeneous nature. 
The relative orientation of a pair of photographs by the least square adjustment ( by computa 
tion ) based on the parallax measurements at 6 'points measured by means of ¿«/-movement in the 
aerial triangulation instrument is an improvement. Normally, measurement of parallax at 5 points 
is sufficient for achieving a relative orientation but the sixth measurement though redundant 
leads to the possibility of a least square adjustment and consequent uniformity in different 
relative orientations. This numerical method of relative orientation based on parallax measurements 
at 6 points gives homogeneous results throughout the strip and is independent of the number of opera 
tors. This numerical method of relative orientation is being used for aerial triangulation since 1961. 
Due to the shortage of aerial triangulation instruments in India, it has been necessary to 
separate the altimetry and planimetry for small topographical scales, i.e., 1 : 50,000. While aerial 
triangulation in space is used for the extension of height control, the planimetric control points are 
extended by radial triangulation. 
The Survey of India is aware of the advantages of the auxiliary instruments such as horizon 
cameras, statoscopes, etc., for the determination of camera orientation elements which will assist in 
improving the speed and accuracy of photogrammetric triangulation in space and plotting and several 
auxiliary instruments are on order. With the arrival of electronic computers in India, experiments 
with analytical aerial triangulation will be commenced.