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Acquisition of Topographic Data by Laser Scanning and Digital Photogrammetry 
Karl Kraus 
Institute of Photogrammetry and Remote Sensing 
Vienna University of Technology 
Vienna, Austria 
e-mail: kk@ipf.tuwien.ac.at 
Extended Abstract 
Data from air-borne laser scanners provide the chance of accurate digital terrain modelling (DTM) in 
forested areas. With the use of airborne laser scanners dense point sets can be provided with a vertical 
accuracy of about ±0.25cm in flat terrain. Depending on the system used, the horizontal linear point 
distances vary from 0.3m to 2.5m. In wooded areas a large portion of the points is not on the ground 
surface. These points lie much higher, as the laser beam can be reflected in the tree tops. These are the 
so-called vegetation points. To find the real ground surface an interpolation technique for this 
asymmetrical error distribution is required. 
For this task we proposed to use the well-known linear prediction combined with robust estimation. 
With this special filtering and interpolation method an automatic classification of the laser points into 
terrain and vegetation points is possible. The experiences made up to now show, that this method can 
be used even for a penetration rate of about 25%, if the vegetation and terrain points are mixed 
thoroughly. 
In the future we will concentrate on the modelling of the surface through the tree tops. This surface 
can be brought into relation with the terrain surface. The volume between these two surfaces is of 
importance in forestry. For this aim laser scanner data recorded with first and last impulse are of 
special interest. 
The derived contours of a laser scanner DTM are poor in geomorphological detail. Further research 
and developments are necessary to find the right combination of laser scanner data and other data 
sources, especially digital photogrammetry. The result will be a high quality DTM for large scale 
applications. 
References 
K. Kraus, N.Pfeifer: Determination of Terrain Models in Wooded Areas with Airborne Laser Scanner 
Data. Photogrammetry and Remote Sensing, 53 (1998), pp. 193-203. 
N. Pfeifer, K. Kraus: Restitution of Airborne Laser Scanner Data in Wooded Area. Geo-Information- 
Systems, 2 (1999), pp. 18-21.