Airborne Altimetry of Glaciers and Associated Errors

Airborne surface elevation profiles have been measured down about 110 glaciers in Alaska, northwest Canada and a few in Washington. They were chosen to represent elevation and area changes of glaciers in different regions; many were profiled in the mid-1990s. Most of the glaciers were profiled down the centerline, some were profiled along more than one track, and some had terminus profiles. Differencing these surface profiles and those drawn along the same ground track on ca. 1950s small-scale maps allow thickness changes over this time interval to be determined. Repeat profiles were done in the late 1990s and/or the early 2000s, allowing thickness changes to be determined over one or more recent time intervals. The along-track thickness changes were then extrapolated over the entire glacier surface to estimate glacier-wide volume and area changes. Further extrapolation of these changes can then be used to investigate glacier change as a measure of regional climate change and provide glaciological input to measured changes of sea level over different time periods. However, all of these various measurements and extrapolations have associated errors that contribute to the overall error budget of glacier volume changes. These errors include those related to: - airborne profiling - 1950s topographic maps - extrapolation of the longitudinal profiles to the entire glacier - temporal area changes - extrapolation of measured volume changes to unprofiled glaciers in specific region The combination of these errors affects the investigation of climate-glacier change interaction, as well as errors in the contribution of glaciers to sea level. It must be emphasized that most of the errors described here are not unique to glaciers in Alaska and northwest Canada, nor are they restricted to profiling instruments that do not scan along a swath. Such errors are directly applicable to airborne profiling measurements and their extrapolation to changes of the large ice sheets.