[Back]

C. Briese, H. Lehner, N. Pfeifer, A. Roncat, R. Sailer:
"Radiometric Calibration of Airborne Laser Scanning Data for the Analysis of Alpine Cryosphere and Periglacial Environment";
Poster: Managing Alpine Future II, Innsbruck; 2011-11-21 - 2011-11-23.

In the last years airborne laser scanning (ALS) became a standard technology for the acquisition
of topographic data. Due to its ability to accurately and densely sample the terrain
surface it became a widely used technique for the generation of digital terrain models. For
the analysis and classification of the topography, geometric criteria derived from the acquired
3D point cloud are typically used. Moreover, ALS systems deliver the amplitude of each echo
(often referred to as intensity) in addition to the 3D position. In contrast to standard discrete
echo ALS systems, advanced full-waveform ALS systems allow to digitise the whole return
signal and they are therefore additionally able to estimate the echo width of each acquired
3D point. Based on these additional physical observables (amplitude and echo width (either
observed or estimated)) the return power of the target can be calculated. This allows, next to
an analysis of the geometry, a radiometric interpretation of the observed surface.
In order to study the radiometry acquired by ALS sensors a correction of the point-wise influencing
factors (e.g. range, angle of incidence, surface characteristics, atmosphere, etc.) has to
be performed. By using reflectance values of in-situ surfaces within the area of interest, e.g.
determined with a RIEGL (RIEGL Laser Measurement Systems GmbH, Austria) reflectometer
and Spectralon® targets, radiometric calibration enables to convert the amplitude and echo
width into absolute radiometric values which describe the characteristics of the observed
surface. With the help of this radiometric calibration the gained radiometric values can be
compared between different flight strips, different flight missions, and even different sensors
(if the used laser wavelength is identical). With this procedure, classification becomes
independent of individual sensor and mission parameters.
Within the project C4AUSTRIA (Climate Change Consequences on the Cryosphere - funded
by Austrian Climate Research Programme (ACRP), Österreichischer Klima- und Energiefonds)
ALS data are applied to analyse radiometric information gained from ALS data for the classification
of the cryosphere and periglacial environment. Within C4AUSTRIA two ALS campaigns
are scheduled and next to the geometric information the calibrated radiometric values will be
compared. In this contribution, the results of absolute radiometric calibration stemming from
the first ALS campaign carried out in 2009 will be presented. Next to the practical workflow
of the whole radiometric calibration procedure the results of the rock glacier Hochebenkar,
located about 5 km south of Obergurgl (Ötztal Alps, Tyrol, Austria), will be presented. Next to
a study of the relative accuracy of the calibrated radiometric values, the resulting radiometric
image mosaic (calculated from multiple ALS strips) will be analyzed. It will be shown that
the study of the radiometry allows to classify different ice and surface types. Based on these
results, it will be demonstrated that calibrated radiometric information from ALS data can
be used to support the documentation of the actual status of these complex cryosphere and
periglacial environments.
Under the assumption that permafrost surface changes are detectable by multi-temporal
ALS data, modellers, stakeholders and decision-makers will benefit from these radiometric
classifications. C4AUSTRIA provides on one hand reliable information about climate change
consequences in the cryosphere and periglacial environment and on the other hand a comprehensive
methodological base for further investigations.
Acknowledgements: The ALS data series was compiled on base the ACRP (Austrian Climate
Research Program) C4AUSTRIA (project number: A963633), the kind contribution of the
Tyrolean Government (surveying department) and the alpS - Centre for Climate Change
Adaptation Technologies project MUSICALS. We are very grateful for the valuable support.