GLAS/ICESat (Zwally et al., 2002) was the first laser altimeter to fly on a spacecraft in Earth orbit. The grids available here were generated from the first two years of data (2003 Feb to 2005 May).
The ICESat elevations grids for Greenland and Antarctica are available from NSIDC with elevations relative to WGS84 ellipsoid and the EGM96 geoid. The NSIDC ICESat page has a full description of these grids. All grids on this website have elevations relative to the EGM96 geoid. The elevations grids are identical to the ones at NSIDC.
The following should be used to cite these data: Zwally, H. J., J. P. DiMarzio, and A. C. Brenner, 2012. GLAS/ICESat Antarctic and Greenland Grids, Digital media.
Links are provided to download the images in either JPEG or POSTSCRIPT format. POSTSCRIPT images are gzipped. Image ranges are limited to display the range of the majority of the data well. Actual limits of values in the grids may be outside the limits shown on the plots. A spike at either end of the histogram colorbar is an indication of this but in some cases the histogram bin size is small enough that the spike may not be visible. Also, the height of these endpoint peaks was limited to the maximum value in the histogram so they would not dominate the figure.
Image axes are labelled with the polar-stereographic x and y coordinates.
For the derivation of the DEMs see section 4 (Data Acquisition and Processing) of the NSIDC ICESat page.
The slope, azimuth, and directional slope (dz/dx and dz/dy) grids were generated from grids of elevation relative to EGM96, but given the scale of the grids (500m or 1km), these data can be considered independent of EGM96. Elevations relative to WGS84 or Topex/Poseidon would have given the same results with data to within the uncertainties in the data.
Latitudes and longitudes were computed using standard equations for the polar-stereographic projection (Snyder, 1982).
The directional slopes dz/dx and dz/dy were computed as central differences where possible:
dz(xi,yj)/dx = (z(xi+1,yj)-z(xi-1,yj))/(xi+1-xi-1)
dz(xi,yj)/dy = (z(xi,yj+1)-z(xi,yj-1))/(yj+1-yj-1)
and by forward differences
dz(xi,yj)/dx = (z(xi+1,yj)-z(xi,yj))/(xi+1-xi)
dz(xi,yj)/dy = (z(xi,yj+1)-z(xi,yj))/(yj+1-yj)
or backward differences
dz(xi,yj)/dx = (z(xi,yj)-z(xi-1,yj))/(xi-xi-1)
dz(xi,yj)/dy = (z(xi,yj)-z(xi,yj-1))/(yj-yj-1)
along the edges or where data were missing.
The magnitude of the slope, θ, was then computed from
tan2θ = (dz/dx)2 + (dz/dy)2
and the aziumth (Φ') relative to the polar-stereographic coordinate system was computed from
tan Φ' = (dz/dy)/(dz/dx)
This was stored as an angle measured clockwise from the -y (upward pointing) axis. To convert the azimuth Φ1 = Φ (x1,y1) to an angle relative to north, start with the latitude (L) and longitude (b) of the points at (x1,y1) and (x1,y2), where y2=y1-1. Using the spherical trigonometric sine and cosine laws,
cos c = cos(90-L1)cos(90-L2) + sin(90-L1)sin(90-L2)cos(b2-b1)
sin B = sin(90-L2)sin(b2-b1)/sin c
and the azimuth measured clockwise relative to north is
Φ = Φ1 - B