Glacier Monitoring from Aster Imagery: Accuracy and Applications

The optical sensor ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) on board the Terra satellite offers new possibilities for worldwide glacier monitoring. In this paper, its capability and accuracy for selected glaciological applications is evaluated. (a) ASTER’s along-track stereo sensor allows for photogrammetric DEM generation. Here, we apply PCI Geomatica Orthoengine for that purpose. A vertical DEM accuracy of approximately ±60 m RMS was found for complex high-mountain topography with maximum errors of up to 500 m. For smoother terrain an accuracy of roughly 15 m can be expected. (b) Automatic glacier mapping using the ASTER bands 3 (15 m resolution) and 4 (30 m resolution) revealed no significant differences to corresponding work based on Landsat TM bands 4 and 5. (c) The flow field for Tasman glacier, New Zealand, was successfully derived from repeated ASTER orthoimages. The striking results suggest that ASTER data might help to include ice flow into global glacier monitoring from space. (d) Combining above approaches of exploiting ASTER data with GIS-models represents a valuable tool for integrated hazard assessments from space: the outburst flood from the moraine lake Dig Tsho, Khumbu Himal, of 1985 was reconstructed from an ASTER DEM with good agreement to the actual flood. INTRODUCTION Presently, the USGS-led project Global Land Ice Measurements from Space (GLIMS) aims at compiling a complete satellite-derived inventory of land ice masses on earth (1). For that purpose it mainly relies on optical data from Landsat ETM+ (Enhanced Thematic Mapper) and ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer, on board the Terra satellite). Whereas the application of Landsat data to glacier mapping and monitoring is widely evaluated (e.g. 2, 3, 4), little is known about the potential use of ASTER imagery for that purpose. It is, therefore, the objective of this study to evaluate the benefit from ASTER data for glacier studies by accuracy assessments and application studies: (a) an automatic DEM from ASTER data is compared to one derived from aerial photogrammetry. Such DEMs are of crucial importance for the accurate ortho-projection of ASTER imagery which is necessary for overlay of repeated imagery and other geo-information. In addition, ASTER-DEMs allow for geomorphometric modelling which is able to supplement spectral analyses; (b) multi-spectral glacier classification from ASTER data is compared to glacier mapping from other sensors, such as Ikonos (1 m) and Landsat (30 m); (c) the potential of repeated ASTER imagery for digital ice velocity measurements is evaluated; (d) combination of above techniques represents a powerful tool for assessing the potential of glacierrelated hazards, as exemplified for glacier-lake outbursts. Proceedings of EARSeL-LISSIG-Workshop Observing our Cryosphere from Space, Bern, March 11 – 13, 2002 44 EARSeL eProceedings No. 2 Since the year 2000 imagery from ASTER is available for global observation of land ice. ASTERs spectral and geometric capabilities include 3 bands in VNIR (visible and near infrared) with 15 m resolution, 6 bands in the SWIR (short-wave infrared) with 30 m, 5 bands in the TIR (thermal infrared) with 90 m (Fig. 1), and a 15 m resolution NIR along-track stereo-band looking 27.6° backwards from nadir (Fig. 2). The stereo band 3B covers the same spectral range of 0.76 μm 0.86 μm as the nadir band 3N. The orbits of Terra and Landsat are the same, but ASTER swath width is 60 km, i.e. a third of the 180 km for Landsat. To image targets over the full swath width of Landsat, ASTER can be pointed ±8.5° cross-track in between individual overflights (5, 6). Of special interest for glaciological studies are the high spatial resolution in VNIR, the stereo-, and the pointingcapabilities of ASTER. 2.0 5.0 10.0 1.0 0.5 100 0 A tm o s p h e ri c t ra n s m is s io n ( % ) Wavelength (μm) 1 2 3 4 5 9 10-12 13 14 ASTER VNIR SWIR TIR