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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004 
An example of the ranking Strategy is given in table 1 for the 
height difference between matched points and preliminary 
DEM. The theoretical height accuracy o; is derived from height 
above ground. 
  
Height difference to Ranking of points 
reliminary DEM outside glacier 
Ranking of points 
on glacier area 
  
<0.5*c, 2 classes up 1 class up 
  
0.3%g, 10 1.0%g, I class up Same class 
  
1.0*c; to 3.0*o, Same class Same class 
  
3.0*c; to 10.0*c; 1 class down Same class 
  
2 classes down | class down 
  
  
  
> 10.0*c, 
  
  
Table 1. Example of point ranking 
2.4 Verification and valuation of results 
After ranking based on quality estimation the matched DEM 
points are sorted in different files depending on the estimated 
quality class. Thus the “bad” points (usually classes 4 and 5) 
quickly can be re-measured at an analytical plotter, while 
“good” points (usually classes 1 to 3) can be checked at digital 
stereo workstations. Time consuming correction of points at the 
digital stereo workstation is reduced significantly. 
Miss-classifying of points mainly depends on the quality of the 
preliminary DEM. As DEM capturing in glacier areas often 
means multi-temporal measurement, high quality preliminary 
DEM is available in many cases. 
Typical miss-classified points are points at the glacier tongue, 
often classified as "bad" points caused by really existing terrain 
changes (height and orientation). On the other hand these areas 
are very important for glaciology and therefore best verification 
of these points is of high interest. 
In total the presented semi-automatic matching strategy 
including the tool of knowledge based point analysing is about 
five times faster than traditional analytical point measurement 
and about two times faster than conventional point verification 
at digital stereo workstations. 
The geometric aspects used for searching of best suited stereo 
models could be used further on for efficient point capturing in 
image blocks with large overlaps (up to 80%) as usually 
available when capturing images from low altitude in 
mountainous areas. Building additional stereo models with the 
images after next, the height accuracy can be improved for 
valley regions. The allocation of the DEM points to the best 
suited stereo models then could be automated by the presented 
software tool. This tool could also be expanded to select best 
suited images for ortho mosaics. 
3. LASER SCANNING IN GLACIER AREAS 
During the last decade airborne laser scanning has made a 
decisive technical improvement and has become a standard and 
well-accepted method for the acquisition of topographic data 
for many applications. First investigations in high mountain 
areas have shown good results (e.g. Favey, 2001). Also 
accuracy evaluations in comparison with aerial image matching 
have been made (Baltsavias et.al. 2001), 
The results presented in this paper are based on data captured 
for the EU-funded OMEGA project. Its main objective is the 
development of an Operational Monitoring system for 
European Glacier Areas, aiming to offer accurate and up-to- 
755 
date information based mainly on remote sensing technology 
(Pellikka et al. 2001). 
One major aspect for the achievement of the objective is the 
generation and utilisation of digital elevation models from 
spaceborne and airborne data. In OMEGA digital elevation 
models of following sources are constructed: VHR satellite data 
(IKONOS, EROS), aerial photography (analogue and digital), 
terrestrial photography, airborne SAR, airborne laser scanning. 
The method of DEM capturing by airborne laser scanning is 
expected to reach high accuracy for mountainous applications 
and will be therefore introduced and investigated in detail. 
3.1 The principle of airborne laser scanning 
Airborne laser scanning integrates a Global Positioning System 
(GPS) receiver for determining the position of the sensor, an 
Inertial Navigation System (INS) for determining the attitude of 
the sensor and the scanning system using laser technology. All 
components are time-synchronized. Different technical 
solutions for the laser scanning system exist. With the laser 
scanning system used in this study (Optech Airborne Laser 
Terrain Mapper - ALTM 1225) the laser beam is swept 
perpendicular to the ground track, thus producing an even 
distribution of data points. The density and distribution of the 
data points depend on the scan angle, the scan frequency, the 
height above ground, the aircraft speed, the swath overlap and 
the reflectance characteristics of scanned surface. A 
comprehensive overview on laser scanning technology is given 
by Ackermann (1999). The high accuracy and dense coverage 
(more than 500.000 points per km? are possible) give the 
possibility of generating high-quality DEM. 
3.2 Data acquisition and pre-processing 
In OMEGA the possibilities and limitations of airborne laser 
scanning as an independent method for glaciological 
applications are investigated and evaluated (Geist et al. 2003). 
For this purpose 10 data acquisition flights were organised by 
the Institute for Geography, University of Innsbruck and carried 
out between 10/2001 and 9/2003 over glacier areas in the Rofen 
valley, Otztal Alps, Austria. 
The laser scanner data acquisition was conducted by TopScan 
GmbH, Rheine, Germany, with an Optech ALTM 1225 laser 
scanner (see table 2). 
  
  
  
Measuring Frequency 25.000 Hz 
Scanning Angle +/- 20° 
Scanning Frequency 25 Hz 
  
1064 nm 
2000 m 
Laser Wavelength 
Max. operating altitude 
above ground 
  
  
  
  
  
Table 2. Parameters of the Optech ALTM 1225 laser scanner 
After the acquisition the raw data were pre-processed by 
TopScan. The pre-processing comprises the determination of 
the absolute position of the laser scan system during the flight 
by analysis of the time-synchronized GPS and INS data, 
calculation of the relative coordinates, System calibration and 
finally calculation and delivery of the coordinates in WGS 84 
format. The primary product of data acquisition are coordinates 
(x, y, Z) of single reflections. A detailed overview on the pre- 
processing steps is given by Wever and Lindenberger (1999). 
Data of two permanent GPS receiving stations (Krahberg and 
Patscherkofel) were used for the differential correction. The