of photogrammetry : 837), on suitable faces of 
either rock outcrops, hut or tree serve best as 
the control points. 
These,besides being very convenient to establisn 
in the field, also provide accuracy and consistency 
during model orientation in the stereo-plotter. 
The ground survey of these permanent control points 
can be undertaken independent of photography. 
Further, in case a fresh photography of the area 
becomes necessary, repeat survey of such control 
points is not required. 
If the area to be surveyed does not have suita 
bly located objects for establishing permanent 
control points, then temporary control points 
need to be established. A white painted round 
ball fixed on the mounting rod of the reflector 
of the Distomat (Fig.l) forms a suitable temporary 
control point. An advantage with such an arrange 
ment is that the photography and the survey of 
control point can be done simultaneously. Even 
a temporary control point should be located, to 
the extent possible, over some fixed object with 
a centering mark, so that if repeat photography 
becomes necessary, the temporary control point 
can be placed at the same location, thereby avoid 
ing a fresh ground survey. 
In case the area to be mapped falls in more 
than one stereo-pair then it is required to have 
at least two control points common in adjoining 
stereo-pairs. The above described temporary control 
point, being round in shape is equidimensional 
and if located at vantage open places would be 
visible from different places and hence would 
form an ideal tie point. 
2.2 Photographic Camera Choice 
In course of photography it has been noted that 
in some cases a camera of 200 mm focal length 
would have been appropriate in place of the availa 
ble Camera of 100 mm focal length. Since on account 
of topographic restrictions in field, the camera 
stations had to be located at a farther distance 
than required. As a result the intended area was 
covered only in a part of the photoframe. In such 
cases if a larger focal length Camera is used 
then the required area would be covered in full 
photoframe, providing a larger scale photography, 
which in turn would result in higher resolution 
and greater accuracy. Therefore, it is desirable 
that the photographic unit contains Cameras of 
different focal lengths so that depending upon 
the available locations for photography, the appro 
priate camera may be used. Since the orientation 
system remains the same for these Cameras, only 
one oreintation system is sufficient for the unit. 
In most of the cases only one Camera was used 
for photography. The stereo-pairs were obtained 
from two Camera stations by interchanging the 
position of the target and the Camera. However, 
in case of glacier surveys two Cameras were used 
for obtaining the stereo-pairs at the same instant 
of time, since on glaciers the weather conditions 
fluctuate rather rapidly. Unmounting, packing 
and shifting of the Camera from one station to 
the other, through rugged glaciated terrain, and 
resetting it at the second station involves good 
amount of time resulting in variations in photogra 
phic conditions. 
However, it was noted that orientation of model 
on the stereo-plotter was much easier and rela 
tively more accurate when only one Camera was 
used for the photography. It was perhaps due to 
the fact that the elements of inner orientation 
remain same when only one Camera is used. 
2.3 Photographic prints in field 
It is desirable to get photographic prints of 
C 
Figure 1. Temporary control point (a), fixed on 
mounting rod (b), of reflector prism (c) 
the stereo-pair in the field itself so that control 
points are properly identified and numbered to 
avoid any confusion during model orientation in 
the laboratory. Further stereo-pairs can be inter 
preted in the area itself supported by field checks 
and collection of ground truth data. Thus revisit 
to the area for the purpose can be avoided. For 
photo-interpretation in field a pocket mirror-ste 
reoscope has been found to be very convenient 
and handy. 
In remote, inaccessible areas it is not possible 
to have the facility of a photolab, therefore, 
an improvised, handy photo-printing technique 
was adopted. The standard ready-made re-agents 
were used for the photographic work. Developing 
of negatives is a simpler work, but the main pro 
blem comes in making prints which require a contro 
lled diffused light source. For making prints 
the photographic paper and the negative were placed 
between two ground glass plates, it was exposed 
by a light source from a three cell torch whose 
transparent glass was replaced by a ground glass. 
After one or two trials it became possible to 
get sufficiently good quality prints. Duplicate 
exposures of each area was obtained, one for deve 
loping in the field and the other for processing 
in the laboratory. 
The main consideration for applying terrestrial 
photogrammetric technique for the selected case 
studies has been the constraints inherent in conven 
tional survey techniques for accurate mapping of 
such areas. Besides, the technique has distinct 
advantages of speed, economy and precision over 
conventional survey techniques. 
3.1 Escarpment slope mapping of Supa dam 
A concrete dam was under construction across a 
552 
3. SELECTED CASE STUDIES 
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