Geosat GM Altimeter Data: 12/29/92

The high density Geosat/GM altimeter data south of 30 S have finally arrived. In addition, ERS-1 has completed more than 6 cycles of its 35-day repeat track. These data provide a dramatically improved view of the marine gravity field.

The files in this directory contain global marine gravity anomalies gridded on a Mercator projection (see Sandwell and Smith, EOS trans. AGU, v. 73, p. 133, Fall 1992 AGU meeting supplement). The grid was derived from the following data sources:

Seasat  

Used in areas north of 30 S latitude. Profiles within 10 km of a Geosat/ERM track were excluded.

Geosat/ERM  

Average of 62 Geosat Exact Repeat Mission profiles.

Geosat/GM  

Recently declassified Geosat Geodetic mission data south of 30 S. The original Geosat/GM GDR's can be obtained on CD-ROM from:

National Oceanographic Data Center
User Services Branch
NOAA/NESDIS E/OC21
1825 Connecticut Ave.
NW Washington, DC 20235
Tel. 202-606-4549
Internet: services@nodc2.nodc.noaa.gov

ERS-1  

Fast delivery IGDR's obtained from Bob Cheney at NOAA. Six, 35-day repeat cycles were used in the grid.

All of these data were gridded using the method described in: Sandwell, D. T., Antarctic marine gravity field from high-density satellite altimetry, Geophys. J. Int., v. 109, p. 437-448, 1992. The method was adapted to accommodate multiple satellite inclinations (Sandwell, D. T., A detailed view of the South Pacific from satellite altimetry, J. Geophys. Res., v. 89, p. 1089-1104, 1984).

The gridded data are stored in an integer*2 format without any header or record information.

world_grav.image  

A 4224 by 7800 grid of 2-byte integers = 65894400 bytes. The gravity anomaly units are 0.1 milligal. The Mercator projected image spans longitudes from 0 E to 390 E and latitudes from 72.006 N to -72.006 N. The center of the upper left grid cell (i.e. the first integer in the file) is located at 71.998 N, .025 E. Longitudes increase with a 1/20 degree spacing. The The center of the last integer in the file is located at -71.997 N, 389.975 E.

The files can be accessed either with the program img2xyz or the GMT program img2grd. In addition, it can be used with image processing programs such as GIPS.

img2xyz  

A Fortran/C program to extract a sub area from the global gravity grid and write an ascii file of longitude, latitude, gravity anomaly values. This file can be gridded using any standard gridding program such as the GMP programs xyz2grd or surface.

img2grd  

GMT users can make a .grd file of a sub region with img2grd. Install img2grd.c in your gmt/src directory, and make it like any other GMT program. Install img2grd.l in your gmt/man/manl directory. Then you can use img2grd to extract a sub-region from world_grav.image directly as a .grd format file. It will still be in Spherical-Earth Mercator projected space, however. See EXAMPLES on the img2grd man page for advice on how to work with this.

gips.head  

A GIPS header to attach to the front of world_grav.image. (GIPS is an image processing package written by Peter Ford at MIT. It is primarily used for Planetary applications.)

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