The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008
The ground control station also includes a flight simulator,
which allows the simulation of the real flight to verify the
predefined flight path (see Figure 2).
3. RANDA
3.1 Project Aims
The focus of the geological project Randa is to characterize the
large-scale rockslide in Randa (Wallis, Swiss Alps; see Figure
3). This rockslide will be used as an example of the ultimate
evolutionary stages corresponding to the complete development
of the basal shear surface(s) and internal rock mass deformation.
In order to understand this failure stage in a better way, the
interrelationships between pre-existing tectonic fractures,
subsequent tensile fracturing and shearing of intact rock bridges
will be investigated. Furthermore, for the understanding of the
rock mass structure and kinematics during this evolutionary
stage, the basal shear planes must be identified, the internal
rock mass deformation has to be further studied, and the 3D
time-dependent displacement field has to be measured more in
detail using the UAV-image data and the generated DSM
(Randa, 2008).
Figure 2: Screenshot of the ground control station software
showing the flight lines of the Randa data acquisition. In off
line mode the software can be used for the simulation of the
flight. The orange shading shows the lower part of the Randa
rock cliff.
Figure 3: The Randa rockslide failure (Wallis, Switzerland).
The lower part is situated 1400 a.s.l. The height difference from
the bottom to the top is more than 800m.
Therefore, a high-resolution digital surface model (DSM) with a
spatial resolution of 10-20cm had to be generated and oriented
images with a footprint of 2-5cm in object space had to be
acquired. Because of the non-nadir case and a difficult terrain, the
standard aerial flight planning procedures could not be applied.
Furthermore, the sight was acquired by a manned helicopter in
November 2007, using the Helimap system, combining laser
scanning and oblique images (Vallet, 2007). The system is based
on a Riegl LMS-Q240i-60 laser scanner for point cloud
generation and a Hasselblad HI camera with a Dos Imacon
Xpress 22Mpix back, which is normally used for texture mapping.
Using the two comparable systems, the autonomous flying model
helicopter and the manned helicopter, at one site, allowed the
analysis of the performance of the systems. Moreover, it is
possible to compare and integrate the two data sets.
3.2 Flight planning
Before doing the flight, the main parameters of the autonomous
flight were defined. For the Randa rockslide we decided to use the
Nikon D2Xs, which has a CMOS-sensor with 4288x2848 pixels
using a 50mm lens (Nikon, AF NIKKOR 50mm 1:1.8D) and a
pixel size of 5.5pm. For the recognition of features with a length
of 10-20cm, an image scale of 1:4500 was selected which results
in a pixel footprint of approximately 3cm. The distance to the cliff
was defined to 230m, which is equal to the normally used flying
height above ground. Finally, the side and end lap were set to
75%. Using this high overlapping along and across strip it was
possible to avoid occlusions and gaps in the image data (see
Figure 6). The flight velocity was defined to 3m/s, while the
shutter speed was 1/1000s. Therefore, the influence of the image
motion was negligible.
After defining these parameters, the most recent elevation model
with the highest available point density was used. Therefore, the
LiDAR data provided by swisstopo was selected. Using this data
set and the available orthophoto (swissimage, swisstopo®)
allowed the definition of the area of interest. Hence, the area was
separated in three sections. The sections were selected using the
inclination and aspect values of the slope.
Figure 4: Our model helicopter during a test flight with the
inclined camera looking perpendicular to the flight direction.
For each particular section a plane was defined in a way that the
average 3D distance of the surface to the plane was reduced to a
minimum. While looking perpendicular to the surface, we
assumed to reduce the occlusion in the images. After the
definition of the plane equation, the normal of the plane was
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