Master Thesis defense by Joanna Fredenslund Levinsen – Niels Bohr Institute - University of Copenhagen

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Master Thesis defense by Joanna Fredenslund Levinsen

Developing a Method for Mapping High-Resolution Surface Elevation Changes of Outlet Glaciers Using Combined Laser Altimeter and Digital Elevation Model Data

Ice surface elevation measurements from satellite and airborne laser altimeters have a relatively low error, but are spatially limited to satellite orbits and flight paths. Photogrammetric Digital Elevation Models (DEMs) have a larger error but provide a continuous surface. We combine the complementary capabilities of these datasets to construct high-resolution (~100 m) maps of surface elevations and elevation changes over rapidly changing outlet glaciers. Such a high resolution is needed to resolve the spatial variability of change over narrow ice streams.

The basic principle of the method is to constrain the DEM surfaces to the altimeter flight lines where they overlap in space and time, both reducing the registration error in the DEM and filling the gaps between altimeter paths. This is done using use laser altimeter data from ICESat and NASA ATM as well as DEMs from the French SPOT 5 satellite. Data stem from the Greenland outlet glaciers Jakobshavn Isbrae and Kangerdlugssuaq, and the elevation changes are estimated from 2007 - 2009 and 2007 - 2008, respectively.

DEMS are registered to the altimeter data using a least-squares approach, followed by subtraction and interpolation of the residuals to yield a corrected surface. Two geostatistical interpolation methods are tested: Kriging and optimal linear estimation. Both yield similar results. By Jakobshavn, the terminus region thinned with -35 to -30 m/yr and the flow channel with approximately -20 to -10 m/yr. The remaining surface was approximately unchanged. By Kangerdlugssuaq, the terminus thinned with -20 to -7 m/yr and the flow channel with -15 m/yr. At higher elevations, the surface rose with approximately 10 m/yr.
The errors are 0.3 - 7 m/yr and 0.2 - 4 m/yr, respectively, and their magnitude is largely dependent on the altimeter data coverage. This information is crucial for airborne altimeter flight planning as such error maps can be used to estimate the spatial density of flights to minimize the uncertainty.

Supervisors: Carl Christian Tscherning, Niels Bohr Institute, University of Copenhagen, Ian M. Howat, School of Earth Sciences, Ohio State University.