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3.1 Mirror Stereoscope ; p
The Mirror Stereoscope (fig. 2) is a device for viewing pairs of
overlapping photographs so as to obtain a "three dimensional
optical impression" (Stereomodel) of the photographed terrain
and objects. The mirror stereoscope provided with binocular
attachment forms the principal equipment in this study. The
mirror stereoscope is a very familiar piece of equipment and will
therefore need no further elaboration here.
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Figure 2. Mirror Stereoscope
3.2 Photogrammetric Needle
The photogrammetric needle is a simple device for marking fine
points on paper prints of aerial photographs. It consists of a hard
plastic stem roughly cylindrical in form and about 10cm long (Fig 3).
The stem carries a sharp needle point at one end, and tapers at the
other; the needle is replaceable.
This equipment is used in marking points by pricking through the
paper prints with the sharp needle at the required locations. i
The fine needle point enables accurate measurement; however it is Figure 3. Photogrammetric Needle
made more legible by circling around with colour grease pencil.
3.3 Lens Magnifier and Plastic Ruler
A typical device for accurate measurement on aerial photographs is
the micro-rule. However, in the absence of the micro-rule an
improvized device has been fashioned out in the present study.
This consists of a transparent plastic ruler about 30cm long, gradu-
ated in centimetres and millimetres, and provided with a lens
magnifer (Fig 4). With this improvized device it is possible to obtain
enhanced measurement of coordinates on aerial photographs, up Figure 4 Lens Magnifier and Plastic Ruler
to the fraction of the millimetre.
Lia Ra "T7 T ij rT TIT
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GRAECE TREE thin
3.4 Aerial Photographs
Aerial photographs as the name implies are taken from airbone platforms, usually aircrafts. In aerial photography the
environment is imaged with radiation in the visible portion of the electromagnetic spectrum within the 0.4-0.7 micrometre
wavelength range. The resulting imagery is a perspective (central) projection of the terrian and objects. For the presen
study, wide angle, black and white (panchromatic) aerial photographs of standard format (23 x 23cm) covering the stud
area were used. Three different sets of photographs, taken in 1969, 1978, and 1988, were involved.
3.5 Overlay Material
Overlay material is used on the photograph to enable annotations to be made during interpretation, thereby protecting th
surface of the photograph. There are different types of overlay materials; kodatrace paper is the one commonly used, bt
acetate paper is better and more transparent, and of course more expensive. Where these are not available tracing pape
could be used as overlay though with difficulty. For the present study tracing paper was used as overlay.
4. PRACTICALPROCEDURE
Erosion mapping necessarily involves a number of interrelated operations, the overall aim of which is to identify, invest.
gate, and present information on erosion within a given portion of the earth's surface. The procedure in erosion mappir-:
may be rather complex, depending on the method used, but invariably includes the following operations among others.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000. 391
