authors and credits
Ice surface elevation and ice thickness data are available for a portion of the West Antarctic Ice Sheet. The investigators utilized a laser altimeter and ice-penetrating radar mounted to a Twin Otter aircraft to survey the ice sheet. Ice surface elevations and ice thickness data, derived from laser altimetry and radar sounding results, are available in ASCII format via ftp.
These data are a result of the Corridor Aerogeophysics of the Southeastern Ross Transect Zone (CASERTZ) experiments of the 1990s. The CASERTZ geophysical surveys were aimed at understanding geological controls on ice streams of the West Antarctic Ice Sheet, ultimately to help assess the potential for ice sheet collapse. Blankenship et al. (2001) used ice surface elevations and ice thicknesses (reported here) to calculate driving stresses across the ice sheet and thus to identify regions of rapid basal movement by ice streams.
Laser altimeter and radar sounding data were collected by a uniquely equipped Twin Otter aircraft, which flew at an altitude of 350 to 1000 m above the surface at an air speed of 120 to 130 knots. Surveys were accomplished with 116 flights in three seasons (1991/92, 1992/93, and 1995/96): 96 flights departed from the CASERTZ field camp and 20 departed from Byrd Surface Camp.
Ice surface elevations were determined by laser altimetry, using a 1000W peak-power infrared unit capable of 1000 pulses per second. Integration of pulses eight times per second allowed for a range determination about every 8 m. In the absence of clouds, range precisions were better than 0.1 m. Range determinations were corrected for aircraft attitude and projected to points on the ice sheet using the absolute position of the aircraft (as determined by GPS).
Ice thickness was measured by airborne radio-echo sounding, using a modified version of the TUD (Technical University of Denmark) radar. Two wing-mounted antennae emitted a 250 ns radar pulse at 60 MHz with a peak power of about 8 kW at a repetition rate of 12.5 kHz. Transmissions were digitized and integrated two to five times per second (i.e., every 12 to 30 m along the track). Processing techniques enhanced the reflection pulse of radar off the base of the ice. In most cases this was the ice-bedrock interface, although in some areas such as the ice shelves the reflection marked an ice-water interface.
Ice thickness was determined by measuring the time between the reflection pulse from the ice surface and the bed return reflection, and converting the time to distance given the radar wave velocity through ice (168.4 meters per microsecond). No firn correction was applied. Ambiguity in determining the precise time of the surface and ice-base pulse, along with variations in firn and ice density, are potential sources of error in the ice thickness determination.
See Authors and Credits.
Two tab-delimited ASCII data files are available.
The file "srfelev.txt" contains ice surface elevations and is formatted as follows:
Season/Flight Year Day Longitude W Latitude S Elevation (m) RTZ2/F20a 1992 8 -123.446438 -82.967063 793.992 RTZ2/F20a 1992 8 -123.387936 -82.969907 799.502 RTZ2/F20a 1992 8 -123.321159 -82.973536 805.261 RTZ2/F20a 1992 8 -123.223722 -82.978829 807.663
The first column provides a season and flight designator (seasons were designated "RTZ2" through "RTZ9", with the last season designated "VTZ1"; after the slash is the flight number "Fxxx"). The second and third columns provide the year and Julian day, respectively, of data acquisition. The fourth and fifth columns provide longitude (in degrees west) and latitude (in degrees south). The sixth column is ice surface elevation in meters above the WGS84 ellipsoid.
The file "icethk.txt" contains ice thickness data and is formatted as follows:
Season/Flight Year Day Longitude W Latitude S Thickness (m) RTZ2/F20a 1992 8 -121.337354 -83.051147 1767.58 RTZ2/F20a 1992 8 -121.286318 -83.052909 1709.12 RTZ2/F20a 1992 8 -121.239291 -83.054516 1664.23 RTZ2/F20a 1992 8 -120.498323 -83.079410 1708.75
The first column provides a season and flight designator (seasons were designated "RTZ2" through "RTZ9", with the last season designated "VTZ1"; after the slash is the flight number "Fxxx"). The second and third columns provide the year and Julian day, respectively, of data acquisition. The fourth and fifth columns provide longitude (in degrees west) and latitude (in degrees south). The sixth column is ice thickness in meters.