The standard method of identifying morphometric
features is to pass a local (usually 3by3) window over the
DEM and examine the relationship between a central cell
and its neighbours, and on the other hand through a water
flow analysis over a DEM (Rieger, 1992)
Pits identification process comes to drainage basin
identification process where the lowest point of basin and
outflow point are not the same. In this case the lowest
point is pit base. The basin identification algorithms
involve a ‘basin climbing’ approach where a basin
outflow point is identified and the basin is recursively
'climbed' until all points flowing from the drainage divide
have been covered.
Fig.l. Pit identification process
There are a number of possible solutions to remove pits
from DEM. This may be achieved by either ‘excavating’
cells that connect the base of a pit to its adjacent
downstreambasin, or by flooding pits until outflow is
redirected. The first method is applied here because of
several reasons:
The results of processing are linear features (sinks) that
may be well included in the interpolation as form lines.
The impact of vegetation heights, which are not filtered
out in the preprocessing stage, is greatly eliminated.
3.1 Laying the pitpath
According to the definition, pitpath is such a path that
starts on a pit base, goes upwards through outflow point
and flows away to its adjacent downstreambasin. It has to
be set in such a way that it passes downwards by its
whole length, (much more details about lying pitpath can
be found in Rieger, 1992)
Fig.2. Pit removal process
4. The practical implementation
The methods of geomorphological improvement of
DTMs based on laser-scanner data are still under test, and
here is only a pit removal introduced.
As test area an area of 91 km2 in Vienna Woods was
taken. The company TopScan Germany made the data
acquisition through an airborne laser scanner ALTM
1020 of Optech Inc. Canada. The whole dataset contains
about 9275000 laser points. The mean distance between
points was 3.1 m. The Institute for Photogrammetry and
Remote Sensing produced DTMs for 360 map sheets at a
scale of 1:1000. The DTM has a grid width of 3.125m
resulting in 160x160 grid meshes pro one sheet. More
details about this project can be found in Kraus (1997)
and Kraus et al. (1997).
As test dataset, the final DTM was taken according to the
new method of interpolation and filtering described in
Kraus 1998. By means of program system MATRIX
originally developed by Dr. Wolfgang Rieger, yet partly
adopted for XX-lfamework in SCOP, the pure raster
elevation model was calculated at a resolution of 1 m. For
visualization purposes shading and contouring are
performed.
Test of a new method was done over an area where as
many morphometric features as possible can be found
within a small area. The Fig .3 shows such an area that
comprises several catchment basins that collect water into
their valleys.
