Directory Identifier: GRAVCD-sandwell
Directory Title: High Density GEOSAT/GM Altimeter Data
Principal Investigator: Professor David T. Sandwell
Contributing Organization: Scripps Institution of Oceanography
                           La Jolla, CA 92093

Summary: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.

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 2-byte integer 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.

CAVEAT:  The preceding description was provided to NGDC with the 
original data contribution.  See ww_grav1.txt.  It has been kept in 
this format to preserve the sense of data lineage.  The formats 
described may not exactly match those on the CD-ROM, which are 
documented consistently within ".FMT", ".HDR" and ".GRD" files.