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.)