progressive sampling with the grid interval of
250 metres.
In the progressive sampling method the measur-
ing mark is driven within a defined boundary
from one sampling point to another sampling
point and an initial sampling patch of nine points
is created. If one of the second differeces in height
is equal to or greater than the threshold value,
second differences are calculated and similarly
the values collected in the second run determine
the need for the third run. After a maximum of
three runs a sampling data of adequate density
which may match the undulations of the terrain
is generated. Keeping the hilly characterstics of
the data set in view the data set was further
upgraded by incorporating the significant relief
features like ridges, drains and break lines.
The DEM data set was then processed by DTM
software wherein the data was converted to
POINT file. It was subsequently changed to a TIN
file, which contains the data of the vertices of
slope triangles as X, Y, Z triplets. Finally from
the TIN file profiles were extracted by converting
to a POINT file (GRID file). The GRID file data
was finally converted to graphics.
The DTM surface (mesh of horizontal and verti-
cal profiles) was then evaluated as given in the
subsequent pragraphs.
6. EVALUATION AND ANALYSIS
6.1 In case of data set A, the large scale photog-
raphy was used to exploit the change in the
contrast in the imagery for interpretation of the
details from the stereo model setup in the photo-
grammetric workstation. The data captured in :
respect of this data set was evaluated by compar-
ing with ground truth. The plot of this data is
enclosed as Annexure I About 9046 of the details
agreed on evaluation. Details are brought out in
para ( 7 ) under findings.
In case of data set B, the output as indicated in
Annexure II was not subject to ground truth.
6.2 The DEM generated was evaluated by com-
paring the DEM with the heights of some promi-
nent points as inpretted from the existing map.
The analysis is given in Annexure - II.
7. FINDINGS
7.1 The data in respect of the following features
could br stereo-digitized with convenience by
using stereo-SPOT imageries in isolation :
1) Roads
ii) Forests limits
iii) River courses
iv) Cultivation
v) Village limits ( Blocks )
7.2 Reference to the photography helped in in-
terpretation of the following details to a great
extent :
i) Cluster of isolated huts
ii) Type of roads
iii) Orchards
iv) Depiction of roads in built up area
7.3 RMSE value in height is about 11 meters.
Keeping in view the sensitivity of the floating
mark in stereo-imagery environment, it is recom-
mended that the DEM be used to generate con-
tours at 50 meters vertical interval.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
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