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Global Gravity Grids, Geoid Height and Gravity Anomaly Profiles
The entire collection of GEOSAT ERM (Nov.’86 - Dec. ’89) data over land and ice regions is held at the National Geophysical Data Center (NGDC). These data will yield reasonable elevation values for land and ice regions of gently varying elevation. This data collection should not be used in regions of highly variable terrain. This satellite altimeter data base contains precise geoid and gravity anomaly profiles which were constructed from the average of 66 repeat cycles of GEOSAT. The data were developed by Professor David T. Sandwell at the University of California in San Diego. The data are contained in two files: (1) geo66asc.bin (2,383,232records) contains the ascending profiles which run southeast to northwest between 72S and 72N, and (2) geo66des.bin (2,397,888 records) contains all of the descending profiles. The dataparameters in addition to time and location are geoid height, gravity anomaly, and uncertainty in gravity anomaly. GEOSAT 66 was updated in 1994 to include the 3rd and last year of data. Thus 66 repeat cycles of data are included in the AVERAGE profile calculation. This satellite altimeter data base was contributed by NOS/Geoscience Laboratory and contains data collected during the first 18 months of the original "Geodetic Mission" of the U.S. Navy Geodetic Satellite (GEOSAT). These digital data are in the form of geophysical data records (GDRs) which are described in NOAA Technical Memorandum NOS NGS-46. The data are observed over a tightly spaced (typically 2 or 3 km at 60 degrees latitude) ground track pattern, and are global in coverage. The Southern Ocean data contained in this subset of the original Geodetic Mission were declassified in 1990 and received at NGDC in mid 1991. GEOSAT GRAVITY ANOMALY GRID SOUTH OF 30 SOUTH K.M. Marks, DC 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, g30_UNIX.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 g30__UNIX.BIN are in "normal" byte order (Sun, Mac, etc.); the equivalent file G30_DOS.DOS is in "swapped" byte order (DEC, PC, etc.). RAPP92: This data base was compiled by Dr. Richard H. Rapp, Ohio State University and was received in April, 1993. The data base consists of the following: One file containing a 0.125 degree grid of free-air gravity anomalies and their standard deviations between +/- 72 degrees latitude. The anomalies in the ocean areas have been derived from a combination of Geos-3, Seasat and Geosat altimeter data and the ETOP05U bathymetric data. Although gravity values are given for land areas they have been, primarily, computed from the OSU91A potential coefficient model that is complete to degree 360. One file containing a 0.125 degree gridded mean sea surface (in the mean tide system), in the same geographic region as the data given in the file above. One file containing 30-minute x 30-minute mean gravity anomalies and geoid undulations (in the tide free system), derived form OSU’s 0.125 degree gridded point anomalies and geoid undulations. One file containing 1 degree x 1 degree mean gravity anomalies and geoid undulations (in the tide free system), as derived from the original gridded point values. Principal gravity parameters include mean gravity anomaly and mean geoid undulations. The gravity anomaly computation uses the Geodetic Reference System 1967 (GRS 67) Theoretical Formula. The data are global in coverage where data are available. SANDWELL: 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 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 of 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, 0.025 E. Longitudes increase with a 1/20 degree spacing. The center of the last integer in the file is located at -71.997 N, 389.975 E.