70 
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)