IMPROVING DIGITAL ELEVATION MODELS OVER ICE SHEETS USING AVHRR-BASED PHOTOCLINOMETRY

Advanced very high-resolution radiometer (AVHRR) images and a radar-al timetry-based digital elevation model (DEM) covering part of the north east Greenland ice stream are combined to create an improved topograph ic map of the area using photoclinometry. In this application of photo clinometry, a DEM is used to establish the photometric relationship fo r two AVHRR images of a snow surface. Slopes from the DEM are compared with AVHRR data that are filtered (i.e. blurred) to the resolution of the DEM to give an empirical photometric determination. This is then used to convert unfiltered AVHRR data into quantitative slope measurem ents of the surface in the along-sun direction in each image, resolvin g features not present (or poorly represented) in the DEM. Co-registra tion of the images is based on the assumption that the two slope field s from the images describe one continuous smooth surface, combined slo pes are then converted to topography. In the test case, the technique adds topographic details with spatial scales of similar to 3 to simila r to 20 km. A comparison of our results with airborne laser elevation profiles demonstrates that the new technique recovers most of the topo graphy that is missed by the DEM. The improved topographic map reveals a ten-fold increase in local surface relief over the ice-stream featu re, and shows the presence of shallow troughs over the shear margins o f the feature.