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Investigation and Improvement of Airborne Laser Scanning Technique for Monitoring Surface Elevation Changes of Glaciers
von Etienne Favey, Vorwort von Hans G Kahle und Vorwort von Armin GrünHeadlines of global warming and melting glaciers predict sustainable climate changes for future generations. How much mass the glaciers actually loose, has been—despite its importance investigated only for a few glaciers, as in-situ measuring methods are time-consuming and usually under-sampled.
Airborne laser scanning is a young technology opening new possibilities for qualitative and quantitative determination of surface elevation changes of glaciers. Its rise was possible through the combination of several modern measuring methods, such as high-precision DGPS positioning in kinematic mode, attitude measurements using inertial Systems, and laser distance measurements to non-cooperative targets.
This work investigates the feasibility and possible improvements of the airborne laser scanning technique with respect to the special circumstances of determining surface elevation changes of glaciers. This consists of an error analysis of the method as well as the scrutiny of its limitations, and its application to remote, alpine areas without the need of in-situ measurements.
The key problem consists in bringing together all necessary elements for georeferencing the laser data, where the quality of each contributing part has to be monitored regarding accuracy and systematic effects. The quality of the GPS trajectory can be degraded by various factors like atmospheric refraction, radio frequency interference, or obstruction of satellite visibility. The reliable identification of the integer-valued carrier phase ambiguities can be hindered or even made impossible by such effects. The attitude Solution was realized using an inertial measurement unit. A separate attitude solution with lower accuracy using a GPS multi-antenna array was developed and used to control and correct IMU drift and offset errors. A self-calibration procedure for determining boresight misalignment angles was elaborated. The used laser scanning system showed increased blunder effects with low received signal power. An approach for detecting and removing blunder was implemented.
Using the laser scanning Solution presented in this work, a height accuracy of 0.3 m could be realized flying over a runway at 500 m above ground. Higher flying height above ground and turbulences impede the realization of this data quality in the mountains; it amounts to about 0.5 m.
In the test area at Unteraargletscher, Bernese Alps, Switzerland, measurements for a temporal analysis were repeatedly made using the laser scanning technique. For the lower parts of Unteraargletscher, digital surface models originating from photogrammetry are available for comparison. The determination of the surface elevation change distribution was shown to be feasible with an accuracy of 0.5-0.7 m.
The areas covered by airborne laser scanning are located between 2500 and 3400m above sea level. For the period 1998-1999. a surface elevation increase of 2-4m was measured. This positive change can be related to the immense amount of snowfall during the winter 98/99.
The coverage of Unteraargletscher with measurements of surface elevation change could be completed up to the remote firn areas thanks to the airborne laser scanning technique. It is available for mass balance and flow modeling calculations.
Airborne laser scanning is a young technology opening new possibilities for qualitative and quantitative determination of surface elevation changes of glaciers. Its rise was possible through the combination of several modern measuring methods, such as high-precision DGPS positioning in kinematic mode, attitude measurements using inertial Systems, and laser distance measurements to non-cooperative targets.
This work investigates the feasibility and possible improvements of the airborne laser scanning technique with respect to the special circumstances of determining surface elevation changes of glaciers. This consists of an error analysis of the method as well as the scrutiny of its limitations, and its application to remote, alpine areas without the need of in-situ measurements.
The key problem consists in bringing together all necessary elements for georeferencing the laser data, where the quality of each contributing part has to be monitored regarding accuracy and systematic effects. The quality of the GPS trajectory can be degraded by various factors like atmospheric refraction, radio frequency interference, or obstruction of satellite visibility. The reliable identification of the integer-valued carrier phase ambiguities can be hindered or even made impossible by such effects. The attitude Solution was realized using an inertial measurement unit. A separate attitude solution with lower accuracy using a GPS multi-antenna array was developed and used to control and correct IMU drift and offset errors. A self-calibration procedure for determining boresight misalignment angles was elaborated. The used laser scanning system showed increased blunder effects with low received signal power. An approach for detecting and removing blunder was implemented.
Using the laser scanning Solution presented in this work, a height accuracy of 0.3 m could be realized flying over a runway at 500 m above ground. Higher flying height above ground and turbulences impede the realization of this data quality in the mountains; it amounts to about 0.5 m.
In the test area at Unteraargletscher, Bernese Alps, Switzerland, measurements for a temporal analysis were repeatedly made using the laser scanning technique. For the lower parts of Unteraargletscher, digital surface models originating from photogrammetry are available for comparison. The determination of the surface elevation change distribution was shown to be feasible with an accuracy of 0.5-0.7 m.
The areas covered by airborne laser scanning are located between 2500 and 3400m above sea level. For the period 1998-1999. a surface elevation increase of 2-4m was measured. This positive change can be related to the immense amount of snowfall during the winter 98/99.
The coverage of Unteraargletscher with measurements of surface elevation change could be completed up to the remote firn areas thanks to the airborne laser scanning technique. It is available for mass balance and flow modeling calculations.