GEOSAT GRAVITY ANOMALY GRID SOUTH OF 30 S

     K. M. Marks, D. C. McAdoo, and W. H. F. Smith

The Geosciences Laboratory, Ocean and Earth Sciences (NOAA), has produced a
digital gravity anomaly grid computed from recently declassified Geosat
Geodetic Mission data, combined with Exact Repeat Mission data, for the
region between 30 S and 72 S latitudes.  The grid spacing is 0.04 degrees in
latitude, and 0.05 degrees in longitude.  The grid file, GEOSAT30.BIN, is a
binary file of two-byte signed integers, stored in raster scan line (bands of
latitude) order.  There are 1051 scan lines with the first line at 30 S and
the last at 72 S latitude.  Each line has 7201 integers with the first
element at 0 E longitude and the last element at 360 E longitude.  Values
equal to 32767 indicate land areas where Geosat gravity is unavailable; all
other values should be multiplied by 0.01 to yield Free-Air Gravity anomalies
in mGals. Data in GEOSAT30.BIN are in "normal" byte order (Sun, Mac, etc.); the
equivalent file GEOSAT30.DOS is in "swapped" byte order (DEC, PC, etc.).

Sample C code to read the file:

	int i,j;
	short int rec;
	double lat,lon;
	FILE	*fp;

	fp=fopen("GEOSAT30.BIN","r");
	for (j=0; j<=1050; j++) {
		lat=-30.00-j*0.04;
		for(i=0; i<=7200; i++) {
			fread((char*)&rec,2,1,fp);
			lon=i*0.05;
		}
	}

/* As a check, rec at i=0 should equal rec at i=7200 in the above */

Sample Fortran code to read the file:

	integer*2 igrav(7201)
	real grav(7201),lat(1051),lon(7201) 

	open(7,file='GEOSAT30.BIN',status='old',form='unformatted',
	recl=7201*2,access='direct')

	do i=1,1051
		read(7,rec=i)(igrav(j),j=7201)
		lat(i)=-30.00-(i-1)*0.04
		do j=1,7201
			grav(j)=float(igrav(j))*0.01
			lon(j)=(j-1)*0.05
		enddo
	enddo

/* Sample programs do not check for land flags */

The files "GEOSAT30.FMT;1" and "GEOSAT30.HDR;1" are placed in the GEOSAT
directory for convenient use of the "FREEFORM" and "GEOVU2"
utilities being developed at NGDC. (These programs were not available
for inclusion on the CD -- watch for an announcement of their release.)