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GEOMORPHOLOGIC IMPROVEMENT OF DTM-s ESPECIALLY AS DERIVED FROM
LASER SCANNER DATA
D. Gajski
Institute for Photogrammetry, Faculty of Geodesy, University of Zagreb, Kaciceva 26, 10000 Zagreb, Croatia
Key word: laser-scanner, DTM, water flow analysis
Abstract
Recent advances in laser-scanning techniques made it to a most attractive method of data acquisition for digital terrain
modeling. This is due not only to the impressive level of automatization but also to the increasingly high density and
precision of the points. Methods of filtering data in a preprocessing stage allow for interpolating a DTM very closely
describing the terrain surface.
Further improvement of the géomorphologie quality of the surface thus interpolated can be achieved by deriving
structure line information of it and introducing it as constraints into a final step of interpolation. A raster type water
flow analysis is described and applied, allowing to derive the structural information needed. The impact of these
constraints is then considered. Applying the method as proposed to DTMs based on data acquisition techniques other
than laser scanning may also be of advantage.
A test area, a part of the Vienna Woods has been chosen. Water flow analysis is performed by SCOP.MATRIX within
the frame of an alpha version of the SCOP_DTM_XX digital modeling system to come.
1. Introduction
Airborne laserscanning provides the means for measuring
polar coordinates i.e. directions and distances between
fixed-wing or rotary-wing aircraft and the reflecting
objects on the earth’s surface. When the outer orientation
of the sensor during the scanning is known, then the
measured polar co-ordinates can easily be converted into
Cartesian WGS84 because of the use of GPS and INS for
determining the elements of outer orientation. To
transform laser points into local (national) coordinate
system, the geoid undulation has to be very well known.
This transformation requires data resampling which
might be done either by an interpolation technique or by
the nearest neighbour method.
Depending on the density of the measuring points and the
width of the target grid, the resampling will cause the
position and elevation errors which can only be ignored
for plain and unstructured surfaces. For high quality
DEMs resampling errors must be minimized - which
means that there should be at least two limes more
measurements available than needed for the target grid.
and interpolation to laser scanner data (such a method is
described in Kraus et al., 1998)
However, the contours derived from a thus filtered and
interpolated laser scanner DTM will have low
géomorphologie quality.
Geomorphological constraints into post-processing of
laser scanner data will be included here.
2. Motivation
The hydrological and geomorphological tradition
suggests that fluvially dominated landscapes rarely
contain pits since the process of water transport and
erosion precludes their development. Hydrological
models that transfer water over and ultimately off a
surface often fail to perform if that surface contains pits
from which water may not be removed. As a consequence
elevation models are often pre-processed in some way to
remove such ‘spurious’ pits. (Wood, 1999)
3. Theory and algorithm
The final quality of DEM based on laserscanning is also
influenced by the shadowing effect, because in built-up or
forested areas a flat viewing laserbeam will reflect mostly
walls or treetops and will rarely reach the ground. In the
postprocessing, shadowed areas have to be recognized
and measured points classified on the basis of points
belonging to groundfloor, as well as those not belonging.
The final quality of DEMs based on laserscanning can be
improved largely through applying a qualified filtering
Pits are areas that lie lower as surrounded terrain surface.
The lowest point of depressions is point that lies in a
local concavity (all neighbours higher) and can be
described by second derivatives as:
d 2 z
dx 2
<0, f?<0
dy
(1)
