Determination of the surface and bed topography in central Greenland

The surface and bottom topography of the central Greenland ice sheet was determined from airborne ice-radar soundings over a 180 km by 180 km grid centered on the 1974 "Summit" site (lat. 72°18' N., long . 37°55' W.), using the Technical University of Denmark 60 MHz ice radar . Over 6100 km of high-quality radar data were obtained, covering over 99% of the grid, along lines spaced 12.5 km apart in both north-south and east-west directions. A ircraft location was done with an inertial navigation system (INS) and a pressure altimeter, with control provided by periodically flying over a known point at the center of the grid. The ice radar was used to determine ice thickness; the surface topography was determined independently using height-above-terrain measurements from the aircraft's radar altimeter. The calculated surface topography is accurate to about ±6 m, with this error arising mostly from radaraltimeter errors . The ice thickness and bottom topography are accurate to about ±50 m, with this error dominated by the horizontal navigation uncertainties due to INS drift; this error increases to about ±l 25 m in areas of rough bottom relief (about 12% of the grid). The highest point on Greenland is at lat. 72°34' N., long. 37 °38' W. , at an altitude of 3233 ± 6 m a.s.1. The ice surface at this point divides into three sectors, one facing north, one east-south-east, and one west-south-west, with each having a roughly uniform slope. The ice divide between the last two sectors is a well-defined ridge running almost due south. The ice is about 3025 m thick at the summit . Excluding the mountainous north-east corner of the grid , where the ice locally reaches a thickness of about 3470 m and the bed dips to about 370 m below sea-level, the maximum ice thickness , approximately 3375 m, occurs about 97 km south-south-west of the summit. The average bed altitude over the entire grid is 180 m and the average ice thickness is 2975 ± 235 m. The ice in most of the south-west quadrant of the grid is over 3200 m thick, and overlies a relatively smooth, flat basin with altitudes mostly below sea-level. There is no predominant direction to the basal topography over most of the grid; it appears to be undulating, rolling terrain with no obvious ridge/ valley structure. The summit of the ice sheet is above the eastern end of a relatively large , smooth, flat plateau, about 10-15 km wide and extending about 50 km to the west. If the basal topography were the sole criterion, then a site somewhere on this plateau or in the south-west basin would be suitable for the drilling of a new deep ice core.