International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004 
  
football field in Zwieselstein (Ötztal/Austria) was surveyed and 
used as calibration area. 
3.3 Investigation of the Vernagtferner data set 
In August 2002 the Vernagtferner, one of the glaciers included 
in the OMEGA project and monitored permanently by the 
Commission of Glaciology of the Bavarian Academy of 
Science, was covered by laser scanning. The final data set was 
delivered several months after the flight date due to geo- 
referencing problems. 
The laser points have been transformed from UTM (WGS84) 
into the Austrian Gauss-Krüger system, since the existing 
control points and photogrammetric data are available in this 
system. For accuracy assessments eight test areas have been 
determined with different material properties (ice, snow, rock, 
debris) and terrain inclination. Within these test areas check 
points have been measured with differential GPS. The results 
are given in table 3. 
  
  
  
  
  
  
  
  
  
  
  
Test area Number of RMS error Offset 
check offset corrected 
points [m] [m] 
Rock 1 70 i023 +0.12 
Snow 59 + 0.08 +0.03 
Debris 171 +0.14 -0.13 
Ice 510 + 0.29 -0.21 
Profile (ice) 40 £0.11 -0.19 
Moraine 78 x 0.16 +0.80 
Rock 2 37 + 0.25 +0.54 
Rock blocks 128 £0.26 +0.78 
  
  
  
  
Table 3. Results of the accuracy check by GPS check points 
As common in high mountain surveying, triangulation points 
are signalised by stone pyramids and usually are used as ground 
control points in photogrammetry. It was possible to locate 
some of these points within the laser data. The positions of the 
top point of the pyramid formed by the laser data mainly 
correspond properly to the position of the bench mark given by 
ground coordinates. For checking the height value, the 
difference to the “nearest neighbour” of the surrounding laser 
points was computed at 7 bench marks. Table 4 shows that the 
heights of the laser points obviously are lower than the 
benchmark heights because the corresponding laser points 
normally don’t match the top of the pyramid. Therefore instead 
of stone pyramids other kind of control information like 
geometrically plane objects (e.g. roofs) should be used (see also 
Kraus, Pfeifer, 1998). However, such objects hardly can be 
found in high mountain areas. 
  
  
  
  
  
  
  
  
  
Bench mark Height difference (m) 
STM 1 -0.70 
STM 2 -0.74 
STM 3 -0.71 
STM 4 -0.75 
STM 6 -0.40 
Gabel (Hut) -0.70 
STM 7 -0.76 
  
  
  
  
Table 4. Height differences at bench marks (stone pyramids) 
A further indication on the quality of laser data is given by 
checking the consistency of overlapping strips. For this reason a 
small test area within overlapping strips was investigated. After 
756 
generating a Im DEM contours with an interval of 0.5 m were 
derived. In figure 3 the contours are superimposed with the 
laser data from adjacent strips. It is obvious that the waves are 
caused by the height differences of the adjacent strips. The 
height difference can be estimated by graphical interpretation to 
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Figure 3. Contours showing “wave effect” 
4. COMPARISON OF IMAGE MATCHING AND 
LASER SCANNING 
Within the OMEGA project an image flight over the 
Vernagtferner glacier has been carried out in August 2003. The 
colour images with a mean image scale of 1: 16.000, taken by a 
standard RMK TOP camera (focal length 154 mm) have been 
used for semi-automatic DEM generation (s. Chapter 2). The 
exterior orientation was reconstructed using some of the bench 
marks, also used for checking the laser data in order to ensure 
. for consistent geo-referencing. This way the laser DEM and the 
DEM from aerial images can be compared, taking into account, 
that there is a time difference of one year. 
4.1 Test area glacier tongue 
A test area was defined, including the glacier tongue and the 
forefront of the glacier. The captured laser data show 
considerably varying point density. Sparse reflectance can be 
recognized especially on the wet ice surface and on the streams 
(see figure 4 and 5). A 2.5 m spaced DEM was generated both 
from the laser data and the aerial images. The shaded relief 
models (Figure 6 and 7) show more details in the laser DEM 
than in the DEM from image matching. 
For a detailed inspection height differences at the DEM grid 
points have been calculated and colour coded (Figure 8). It is 
obvious that there was a considerable height decrease at the 
glacier tongue (up to 5 m) and at the upper right part of the test 
area caused by melting debris covered ice. For accuracy 
consideration only the non ice covered area can be investigated. 
The colour coded height differences in this area show a constant 
shift of about 0.5 m. 
Calculated offsets between matched DEM, laser DEM and GPS 
check points are presented in table 5. Offsets may be caused 
either by an error of the exterior orientation of the aerial images 
or by a systematic error of the laser points. It seems that laser 
scanning is the more accurate method. Exact statements, 
however, needs to evaluate the local behaviour of the different 
height systems, i.e. geometrical heights for GPS measurements 
and orthometric heights for photogrammetric ground control. 
The difference between the height systems, called geoid 
undulation, can vary significantly in high mountain areas. 
Intern 
  
  
Figure 
  
Figure 6 
  
Figure 8.