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Figure 6. Inspection of FWF-ALS echoes in vegetated areas 
with dense understory (red points) and TLS point cloud (green). 
Next to the examination of the echoes that belong to one line of 
sight, the influence of low vegetation on the FWF information 
is analysed. One example for this study is displayed in Figure 6. 
In the upper part of the figure dense brushwood can be found 
below a tree (marked by a yellow rectangle). In the middle of 
the figure a more detailed view of the understory area with the 
red ALS points attached on the TLS observations can be found. 
In the lower part of the figure the digitised FWF information for 
one representative ALS point is displayed in a graph. Compared 
to other echoes from planar extended targets (compare for 
example the echoes of the tree trunk in Figure 4) it can clearly 
be seen that the understory leads to an enlarged echo width. 
This increase of the echo width is caused by the reflection of a 
sequence of small reflecting surface elements at different 
heights and provides therefore a good basis for the automated 
detection of influence of these low vegetation cover on the 
resulting ALS points. In these areas, points that are affected by 
understory will typically lie slightly above the terrain. Without 
the additional knowledge from FWF-ALS these points are very 
As ALS constantly finds new fields of application, special 
demands are increasingly made on the results. Archaeology for 
example needs a high quality separation of terrain and off- 
terrain points to derive detailed DTMs displaying micro- 
topographic variation even under forest canopies. Biology and 
forestry are interested to extract individual trees from the point- 
clouds. During the last years, FWF-ALS turned out to have a 
high potential to meet many of these requirements. However, an 
in-depth understanding of the FWF-information is essential to 
enhance the quality of the DTM and to allow a reliable 
automated interpretation of the acquired data. 
This paper aimed to start the investigation of the complex 
interaction of the laser beam with different types of vegetation 
cover. Part of a forest was scanned by airborne and terrestrial 
laser scanning (Riegl LMS-Q680 and Riegl VZ-400). The 
combined data acquisition took place simultaneously on a calm 
day. Using tachymetry, the data sets were geo-referenced and 
the differences between the ALS and TLS data sets were 
minimized by an adjustment using planar control and tie 
patches. 
The investigation of each individual FWF echo together with 
the derived FWF parameters and the digitised waveform could 
be done within the context of the object that is provided from 
the TLS data. In that way, we could gain interesting results 
especially from densely vegetated areas, which will help to 
improve algorithms for the advanced usage of FWF 
information. 
In the future we want to quantify the accuracy of the geo- 
referencing of the ALS and TLS data in more detail. 
Furthermore, we aim to study the interaction of the laser beam 
with the terrain and the attached objects by further advanced 
visualisations (e.g. by the direct visualisation of the FWF signal 
and the FWF parameters in the 3D view). 
REFERENCES 
Briese, C., 2010. Extraction of Digital Terrain Models. In: 
Airborne and Terrestrial Laser Scanning, Whittles Publishing, 
ISBN: 978-1904445876, pp. 135-167. 
Doneus, M. und Briese, C., 2006. Digital terrain modelling for 
archaeological interpretation within forested areas using full- 
waveform laserscanning. In: M. Ioannides, D. Arnold, F. 
Niccolucci und K. Mania (Editors), The 7th International 
Symposium on Virtual Reality, Archaeology and Cultural 
Heritage VAST, pp. 155-162. 
Doneus, M., Briese, C., Fera, M. und Jänner, M., 2008. 
Archaeological prospection of forested areas using full- 
waveform airborne laser scanning. Journal of Archaeological 
Science, 35, pp. 882-893.