Determining the Antarctic Ice Sheet Grounding Line with Photoclinometry using LANDSAT Imagery and ICESat Laser Altimetry

Team Members:
Jamika Baltrop
MyAsia Reid

Mentor:
Malcolm LeCompte

Keywords:
Photoclinometry, Grounding Line, LANDSAT, Ice Sheet, GLAS

During the last century, ocean temperatures increased by approximately 1 Celsius degree. Long-term observations of portions of the coastal Antarctic ice sheet reveal increasing melt rates thought to be due to the ocean temperatures increase. As a result of the warming, ice sheet margins are observed to be retreating from the edge of the ocean by approximately 1 meter per year for each 0.1°C rise in ocean temperature. The area of the Antarctic ice sheet is thus in a state of flux.
As ice approaches the ocean from its landward, upslope side, it eventually enters the water and begins to float becoming an ice shelf. The relatively warm water at the base of the shelf causes it to melt into the ocean. The “Grounding Line” or GL is considered the point at which the grounded ice, in its continuous movement down-slope toward sea level elevation, enters the water and begins to float. It is more accurately referred to as the Grounding ‘Zone’ because the actual position fluctuates with tides and wave action.
It is difficult to determine the actual size of the Antarctic ice sheet due to the uncertainty in the location of the GL. Making an accurate determination of its location during a narrow temporal window (perhaps a 3-5 year interval) would be useful in estimating the mass-balance of the southern ice sheet simply by providing a perimeter across which the ice enters the water at a rate of 1 meter per year. This can be compared with the amount of precipitation observed to occur over the continental ice sheet.
Photoclinometry software obtained from NASA Goddard Space Flight Center was used to determine the geographic location of the GL. Photoclinometry uses differences in the surface brightness of LANDSAT scenes of coastal Antarctic, corrected for variations in solar illumination angle to determine relative slopes in a scene. The slopes were adjusted to actual elevations by associating brightness levels with actual elevations along scan paths of the GLAS (Geoscience Laser Altimetry System) laser altimeter GL
Terrain brightness was then compared between scan paths to produce an elevation map of a scene. The team derived an assigned portion of the Antarctic Ice Sheet GL along the coastline recorded by a LANDSAT scene centered at 120° West longitude.