"Strezhen" - axis of flow channel) visually illustrate
direction and velocity of ice movement at given
glacier areas (Fig. 4).
Drawing strezhinels is based on the preliminary
constructed vector field. Beginning from the first
point selected by operator vectors interpolation and
automated drawing double line are made. Width of
the double line is proportional to vector length in
that point. Moving along flow motion with given step
a double line of variable width is drawn, that visually
describes the direction and speed of the
flow.Velocity vectors and strezhinels are displayed
on the monitor screen in given scales. A printer may
be used for imagery printing.
A stereoscopic model constructed with the help of
simultaneous photos permits us to obtain
morphometric parameters of the object of interest,
for example, to draw profiles, to determine slopes,
to represent glacier surface in stereoscopic
projection, and so on. It is to be noted, for said
goals high precision exterior model orientation is not
necessary.
Accuracy estimation and conclusion
All measurements carried out at different parts of
pulsing glacier Medvezhy and at inaccessible icefall
with height more than 4000m, in particular, have
confirmed the high efficiency of the method.
Comparison of two versions of the reported method
was made to estimate accuracy obtained from
photoprint measurement results. The first version is
measurement of negatives on a high precision
stereocomparator. According lo previous
investigations the accuracy of this method equal to
0.02 mm in the scale of the photo. The second
version is measurement of scanned photo prints at
the monitor screen.
in both cases, velocity vectors were measured in
fact at the same points. Results of comparison have
shown that discrepancies, as a rule, do not exceed
a value of 0.04 mm in the scale of the
photogram.They agree to one pixel for given
resolution. If we accept that error in the glacier
velocity calculation does not exceed 10%, the
method could be considered as reliable when ice
shift value for period between two aerial surveyings
is not less than 0.4 mm in the scale of the photos
when contact photoprints were used.
Produced experiments allow us to make the
following conclusions:
1. Based upon developed technique it is possible to
carry out aerial photo measurements for studying
302
glacier movement and its morphometry using
contact photoprints from negatives and using
relatively cheap and widely used equipment without
expensive photogrammetric working stations.
2. |t is possible to make stereoscopic
measurements on unsynchronized photos with
sufficient success, however, when glacier has to be
processed high quality of pseudo-stereoscopic
effect is provided only with rather small time
intervals between different surveyings. One-two
weeks interval may be considered as optimal. To
carry out aerial surveying at the same time of the
day, with similar parameters of flight is a very
important factor.
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Fig.3. Vectors of the ice movement surficial velocity
according to icefall part shown at Fig.2. Vectors
scale in meter /day is shown.
References:
Crippen Robert E. 1992 Episodes. Measurement of
subresolution terrain displacement using SPOT
panchromatic imagery. 15, Ne1. 56-61.
Finsterwalder R. 1958. Measurement of ice velocity
by air photogrammetry. Publ. Assoc. Internat.
Hydrol. Scient. Ne47
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996