International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
image flights only a few days per year are available. Laser
scanning with its active sensor can be used also if the light
conditions are to poor for an image flight.
Problem strip overlap:
To cover the Vernagtferner glacier two image strips had to be
flown, while in laser scanning 11 strips were necessary. This
means, that the operating time for the data acquisition is
considerably higher for laser scanning than for an image flight.
In Photogrammetry with automatic aerial triangulation and
bundle block adjustment a well known and operational
technology for the adjustment of image blocks is given. In laser
scanning the adjustment of adjacent strips is not yet solved and
is object of various research projects.
Geo-referencing:
Image orientation in Photogrammetry can be done either by
control points or direct geo-referencing by GPS/INS. A
combination of both strategies leads to the most accurate and
reliable results.
In laser scanning the exact measurement of the flight trajectory
and the inclinations of the system is essential and the accuracy
of the dataset is depending mainly on this issue. In alpine
regions the availability of permanent GPS reference stations
and the knowledge about the geoid undulation often is limited.
Accuracy:
The accuracy investigations based on terrestrial check points
have shown, that laser scanning reaches an absolute accuracy of
0.1-0.2 m in snow and ice covered areas, while a constant offset
up to 0.8 m was detected in test areas located on a rather steep
moraine and in rock blocks. As the offset corrected RMS-value
ranges between 0.1 and 0.3 m, the large offsets are clearly
caused by systematic errors. This is confirmed also by the
detected differences between adjacent laser strips up to 0.5m. In
photogrammetry the accuracy potential of the laser data can
only be reached using image flights with altitudes above terrain
of about 1.000 to 2.000 meters.
5, CONCLUSION AND OUTLOOK
With semi-automatic DEM capturing based on aerial images a
very operational and accurate method for glacier monitoring
exists. Efficiency and accuracy can be significantly improved
by additional considerations like knowledge based point
analysing, adapted flight planning and digital sensors. Besides
surface modelling the images contain a lot of radiometric
information (colour, texture) for context derivation. In snow
and firn areas, however, image textures are often not sufficient.
Airborne laser scanning provides a method which reaches high
point density independently from the terrain texture. Therefore
it offers new possibilities for glaciological research in areas
covered by snow and firn, which is essential for data capturing
in the accumulation period.
Accuracy potential of airborne laser scanning can be in the
range of 0.1 - 0.3 m, but systematic errors of about 0.5 - 1 m
have been detected in the practical results. Further
investigations in direct geo-referencing of the laser data
promise a noticeable reduction of these systematic errors.
Combined application of digital photogrammetry and airborne
laser scanning, based on a homogenous system of GPS-
reference and control points, is suggested as the best suited
method for high accurate glacier monitoring.
6. AKNOWLEDGEMENT
The authors are grateful to the board of the OMEGA project,
funded by the European Commission (EVK2-CT-2000-00069),
for supplying the laser data and aerial images. The Commission
of Glaciology of the Bavarian Academy of Science supported
the field work for the measurement of terrestrial check points.
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