| 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. |