The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
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calculated and with the given flying distance to the surface a 
parallel plane was defined (see Figure 5). In the new plane the 
area of interest was projected and the inclined flight lines for 
the helicopter flight were generated. Finally, using this 
information the image acquisition points were determined and 
transferred to the navigation unit (wePilotlOOO) of the mini 
UAV-system. Due to the inclination of the plane the camera 
was mounted with a pitch angle of 15-30° (respectively for the 
individual sections) on the helicopter (see Figure 4). 
Such precise planning was necessary due to the large distance 
between operator and mini UAV, which limited the visibility of 
the mini UAV. Furthermore, before flying along the predefined 
flight path, the mini UAV was tested at an altitude of 2400m 
a.s.l. in order to ensure that the engine of the helicopter would 
not fail due to low air pressure. This test was important, since 
the helicopter manufacturer set the maximum flying altitude to 
1500m a.s.l. In the following, the results from the first flight in 
Randa and an elevation model extracted from the acquired data 
will be described. 
X [m-10 5 ] 
Figure 5: Zoom-in to one section of the cliff showing the 
LiDAR-DSM with the approximated and parallel plane from 
the flight planning. 
3.3 Field Work 
Since the Randa rockslide covers a height from 1400m -2300m 
a.s.l. the mini UAV-system was tested at the Flueelapass (~2400m 
a.s.l., Graubuenden, Switzerland). For the flight at Flueelapass 
weight dummies instead of the mounted camera were used on the 
helicopter. To enable the evaluation of the performance at such a 
height above mean sea level, the helicopter was started and 
switched to the assisted flying mode. In this mode the current 
parameters of the engine were checked visually and saved on 
board of the helicopter. This specific test showed that the engine 
of our mini UAV-system already achieved the limit for the 
maximal turning moment of the engine. Therefore, to have a 
buffer, we decided to do the first flight only at the lower part of 
the rockslide (1400-1800m a.s.l., see Figure 3), while the upper 
part was also covered by the Helimap flight. 
Since the illumination conditions were acceptable only in the 
morning, we decided to do the flight at Randa in the morning 
hours. In the afternoon strong shadows made the data processing 
more complicated, while a preprocessing of shadow areas had to 
be accomplished. Furthermore, the GPS-satellite availability was 
simulated in the preparation process. Thus, the existing elevation 
model of the surrounding (DHM25, swisstopo®) was integrated 
in a GPS sky plot software, which allowed the calculation of the 
accessible number of GPS and the postulated GDOP (Geometric 
Dilution Of Precision) value. Using this information, it was 
possible to have an approximate a-posteriori value for the 
accuracy of the GPS-position of the helicopter, which was crucial 
in such an environment. 
Given that at the bottom the site a large debris zone is situated 
(see Figure 3), the accessibility of the site is quite complicated. 
Therefore, the start and landing point was defined close to a road 
at the bottom of the debris zone (see Figure 2). The field work 
itself was done in few minutes, while the longest part of the flight 
was the way to go from the starting point to the first image 
acquisition point and the way back. At the end of the first flight, 
the helicopter did a fast in-explainable turn along its own main 
axis. Hence, we decided to stop the test and to evaluate the flight 
data, having already the first high resolution images from the 
rockslide. 
Figure 6: Left: Derived surface model from image matching, Middle: Zoom-in of an UAV-image, Right: Point cloud of Helicopter- 
based LiDAR projected on the derived surface model.