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Remote sensing for resources development and environmental management
Damen, M. C. J.

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
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