Improving digital elevation models over ice sheets using AVHRR-based photoclinometry

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Abstract
Advanced very high-resolution radiometer (AVHRR) images and a radar-altimetry-based digital elevation model (DEM) covering part of the northeast Greenland ice stream are combined to create an improved topographic map of the area using photoclinometry. In this application of photoclinometry, a DEM is used to establish the photometric relationship for 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 measurements of the surface in the along-sun direction in each image, resolving features not present (or poorly represented) in the DEM. Co-registration of the images is based on the assumption that the two slope fields from the images describe one continuous smooth surface. The combined slopes are then converted to topography. In the test case, the technique adds topographic details with spatial scales of ~3 to ~20 km. A comparison of our results with airborne laser elevation profiles demonstrates that the new technique recovers most of the topography that is missed by the DEM. The improved topographic map reveals a ten-fold increase in local surface relief over the ice-stream feature, and shows the presence of shallow troughs over the shear margins of the feature.