Talks and Poster Presentations (with Proceedings-Entry):

L. Eysn, W. Mücke, C. Ressl, M. Hollaus, R. Leiterer, F. Blauensteiner, N. Pfeifer:
"Investigations on the orientation of terrestrial laser scanning point clouds in dense forests";
Poster: SilviLaser 2012, Vancouver, British Columbia, Canada; 2012-09-16 - 2012-09-19; in: "SilviLaser 2012 - Conference Proceedings", (2012), Paper ID SL2012-097, 1 pages.

English abstract:
Vegetation structure was identified to have significant influence on the course of environmental
processes. For complex applications in environmental context, such as radiative transfer
modelling in forests, highly detailed and accurate geometric depiction of the vegetation is
among the basic requirements to ensure as realistic conditions as possible for the modelling
algorithms. Terrestrial laser scanning (TLS) provides reliable means for collecting the required
3D information for this purpose. Especially at ranges not in the immediate vicinity of the
viewpoints, the direct 3D collection is advantageous to the indirect acquisition via photographs,
due to the many occlusions in the forest environment. Study sites in dense mountainous forests
pose a special challenge because of the complexity of the terrain and the vegetation. Detailed
a-priori information is typically not available, and therefore the design of the measurement
campaign often has to be arranged on the spot. Undersampling of the low vegetation and
minimal overlap between scans for reduced time consumption in the field, as well as wind limit
the use of ICP as orientation method of choice. Positioning and setup of reference targets, which
are therefore required for merging the data from different scanning positions, can become a
tedious and time-consuming task.
Within this study we compare two different approaches for the orientation of point clouds from
individual scans acquired with a phase-shift scanner in two densely forested areas in Tharandt,
Germany and Lägern, Switzerland. The study area in Switzerland consists of a steep sloped,
unmanaged, forest with various tree species. The study area in Germany consists of a managed
forest with very old spruce and fir trees. A network of reference points was set up and measured
with a total station for each study area to serve as basis for the investigations. These reference
networks consisted of commercially available planar precision targets and self-designed
spherical targets made from lightweight materials. These spherical targets have considerable
advantages over conventional ones. Due to their size and reflective properties the targets feature
increased visibility under leaf-on conditions, their shape offers high flexibility for the
positioning of the laser scanner and they are designed to stand alone. They can be located
anywhere without the need to mount them on a near-by tree. The planar targets, which display
individual black-and-white patterns, were mounted on trees. The first approach uses all
available spherical targets for orientating the individual scans. In contrast, for the second
method the laser scanner was equipped with two distinct standard reflector prisms for
measurement with a total station. This setup allows direct georeferencing and orientation of
each individually acquired TLS scan without the need for reference targets.
Both methods are compared on the basis of practicability of data acquisition, as well as relative
and absolute geometric accuracy of the resulting point clouds. The set of mounted planar
precision targets is used for evaluation purposes.

Electronic version of the publication:

Created from the Publication Database of the Vienna University of Technology.