Figure 1: A hill shaded view of the terrain model.
time being it is still a costly technique which is not feasible for
large areas; but the technological development may soon bring
new systems which allow to fly higher, and to have greater point
density across track, so that it may be economically possible to
have frequent laser scanner flights for large areas.
1.2 Test site and data
As test site the research forest of the Vienna University of
Agricultural Sciences was used. The forest is positioned
approximately 60 km south of Vienna in hilly terrain with
elevations ranging from 350 to 750 m above sea level. The
vegetation is typical for central Europe. More details can be
found in (Rieger, 1999).
A laser scanner flight was taken during winter time (leafless
period) with last reflected pulse recorded. This flight is necessary
in order to obtain a high quality ground model. From that model
roads can be delineated as is shown in section 2. The same flight
may be used to create digital surface models which include the
building surfaces. The difference between the surface and the
ground models is used to extract buildings in the way described in
section 3. Here, summer flights were used instead of the winter
flight since they were available for the test site. The results may
even be better if the surface model is derived from the winter
data.
2 EXTRACTION OF ROAD BREAK LINES
Digital terrain models (DTM) generated from laser points can be
rather detailed due to a huge number of measured points. Despite,
break lines in the terrain appear smoothed, unless they have been
introduced into the DTM interpolation. Usually, these break lines
are digitised manually in a stereo plotter. In this work we tried to
extract forest roads in mountainous areas from the DTM in order
to introduce them in a new DTM interpolation. Such roads are cut
International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 3W14, La Jolla, CA, 9-11 Nov. 1999
Figure 2: Slope model with position of detailed windows.
in the hill slope, so that in the resulting DTM, the road sides must
appear as sharp breaks.
First a digital terrain model with a grid width of 20 x 20 cm! is
calculated using only the laser ground points by applying a robust
estimator with a skew error distribution function in the program
system SCOP (Pfeifer et al., 1999). Figure 1 shows a hill shaded
map of the terrain. The digital terrain model is used for the
creation of a digital slope model which provides the first
derivative of elevation. At each grid point, the elevation angle of
the surface normal (“steepness”) is given in that model. This
slope model is converted into a digital image, where the grey-
levels represent the steepness of the terrain (figure 2). As no
break lines have been considered yet in interpolating the DTM, a
rather smooth slope model is produced with wide transition zones
between flat areas and steep areas.
The images show two narrow roads in a mountainous, forested
area. Figure 3 draws two profiles perpendicular to the roads. Up
to four break lines may be detected. Beginning at the left side, the
first break is between the hillside and the bank of the road (only
in the right profile). At the two road sides the second and the third
break appear. The fourth break is at the end of the ditch. Not all
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Figure 3: Profiles perpendicular to roads.
International
Figure 4: Two deta
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breaks at the road sides
breaks. Figure 4 shows t
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and appear as bright
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As break lines in the teri
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Unfortunately, standard
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Figure 5: Edges
2.1 Edge Enhancement
For achieving satisfying
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based on the ideas of the
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value of the central pi:
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