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Directory Identifier: GRAVCD-s30s
Directory Title: GEOSAT Southern Ocean (30S-72S. Lat.) Altimeter Data
Principal Investigators: Karen M. Marks and David C. McAdoo

Contributing Organization: NOAA/NOS Geoscience Laboratory
Rockville, MD 20852

Summary:GEOSAT DATA: Gridded Gravity Anomalies for the Southern Ocean

A regularly spaced gridded file of gravity anomalies has been derived for the Southern Ocean, from 30 degrees south latitude to 72 degrees south latitude. These anomalies were computed entirely from altimeter data collected from 1985 through 1988 by the U. S. Navy's GEOSAT satellite. Data from both GEOSAT's primary, classified Geodetic Mission (GM) and its secondary Exact Repeat Mission (ERM), were used. The method used to compute gravity anomalies from these data is described in McAdoo and Marks (1991). The work was performed at the Geodetic Research and Development Laboratory, National Geodetic Survey Division, National Ocean Service Charting and Geodetic Services, National Oceanic and Atmospheric Administration. Gravity anomalies have been calculated at the grid nodes of a regular, orthogonal latitude-longitude lattice. These are anomalies of the vertical component, referenced to the geoid. For most practical purposes, these are essentially the same as Free Air gravity anomalies. The node spacing is 0.05 degrees in geographical longitude and 0.04 degrees in geographical latitude. The data set itself is a list of the anomalies, which were computed at these grid nodes, in units of milligals. The list begins at the northwestern corner (30S, 0E), and proceeds in rows of constant latitude. Each row consists of 7201 values from 0E to 360E. There are 1051 rows arranged from north to south, ending at the southeastern corner (72S, 360E). Although the data set covers the entire annular region between latitudes 30 and 72 South, grid nodes which lie over land or permanent ice shelves have no computed anomaly; these nodes are assigned the arbitrary flag value of -999.00. 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.

Also included are derived temporal composites of GEOSAT ERMs 1- 44. Additional reference materials include National Oceanographic Data Center Environmental Information Bulletin No. 90-3 for GM data, and NODC Bulletin No. 91-X for ERM data. Color, shaded-relief images of these anomalies are portrayed in a companion poster and atlas (Marks and others, 1993).


REFERENCES

Marks, K. M., D. C. McAdoo, and D. T. Sandwell, 1991. Geosat GM data reveal new details of ocean floor. EOS Transactions of the American Geophysical Union, v. 72, pp. 145-149.

McAdoo, David C., and Karen M. Marks, 1991. High resolution marine gravity fields from Geosat Geodetic Mission Data. Journal of Geophysical Research, v. 97, pp. 3247-3260. Mapping the Southwest Indian Ridge with Geosat. EOS Transactions, American Geophysical Union, v. 74, Number 8, Feb. 23, 1993.

Marks, K. M. and D. C. McAdoo, Gravity Atlas of the Southern Ocean, NOAA Technical Memorandum, 1992.

Sample C code to read the file:

int i,j;
short int rec;
double lat,lon;
FILE *fp;
fp=fopen(“S30S.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='S30S.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 */


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