This satellite altimeter data base contains precise geoid and gravity anomaly profiles which were constructed from the average of 44 repeat cycles of Geosat. The data are contained in two files: (1) geo44asc.bin (987755 records) contains the ascending profiles which run southeast to northwest between 72S and 72N, and (2) geo44des.bin (991313 records) contains all of the descending profiles. The data parameters in addition to time and location are geoid height, gravity anomaly and uncertainty in gravity anomaly. For additional information see geosat.txt in this directory.

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.

This 2' geoid height grid for the conterminous United States is the G96SSS model. The computation used about 1.8 million terrestrial and marine gravity data held in the National Geodetic Survey gravity data base in July 1996. These data were augmented by gravity data contributions from NGA (former National Imagery and Mapping Agency (former Defence Mapping Agency)). By means of a Fast Fourier Transform (FFT) technique, high frequency corrections were made to an underlying EGM96 geopotential model through a remove, compute, and restore process. The gravity values are based on the International Gravity Standardization Net 1971 (IGSN71). The geoid heights are referred to the Geodetic Reference System 1980 (GRS80) ellipsoid. Unlike GEOID96, the G96SSS grid does not incorporate GPS on leveled benchmarks. The G96SSS model is a gravimetric geoid in a geocentric, ITRF94(1996.0) reference frame. It is necessary to subtract 12.0 cm from the G96SSS values to obtain the geoid undulation between the best-fit global geopotential surface and the GRS80 ellipsoid (both expressed in a tide free system). Additional information is available at: http://www.ngs.noaa.gov We are particularly grateful to NGA (former National Imagery and Mapping Agency) for their assistance and their data contributions.

The gravity station data (1,522 records) were compiled by the Portland State University. This data base was received in August 1990. Principal gravity parameters include Free-air Anomalies and Complete Bouguer Anomalies (terrain correction applied). The observed gravity values are referenced to the International Gravity Standardization Net 1971 (IGSN 71). The gravity anomaly computation uses the Geodetic Reference System 1967 (GRS 67) theoretical gravity formula. The data are randomly distributed within the boundaries of the greater Portland area.

This 2' geoid height grid for the conterminous United States is the GEOID96 model. The computation used about 1.8 million terrestrial and marine gravity data held in the National Geodetic Survey gravity data base in July 1996. These data were augmented by gravity data contributions from NGA (former National Imagery and Mapping Agency (former Defence Mapping Agency)). By means of a Fast Fourier Transform (FFT) technique, high frequency corrections were made to an underlying EGM96 geopotential model through a remove, compute, and restore process. The gravity values are based on the International Gravity Standardization Net 1971 (IGSN71). The geoid heights are referred to the Geodetic Reference System 1980 (GRS80) ellipsoid. GEOID96 incorporates the reference system relationship between NAD 83(86) and ITRF94(1996.0), the datum offset of NAVD 88, and the contributions from 2951 GPS on leveled benchmarks. Additional information is available at http://www.ngs.noaa.gov/GEOID/geoid.htmlWe are particularly grateful to NGA (former National Imagery and Mapping Agency) for their assistance and their data contributions.

The Central Andes gravity data (6,151 records) were compiled by Professor Gotze and the MIGRA Group. This data base was received in April, 1997. Principal gravity parameters include Free-air Anomalies and Bouguer Anomalies (terrain and mass corrected). The gravity station data have been adjusted to the International Gravity Standardization Net 1971 (IGSN 71). The gravity anomaly computation uses the Geodetic Reference System 1967 (GRS 67) theoretical gravity formula. The data are randomly distributed within the area bounded by 64 - 71 degrees West and 20 - 29 degrees South in the Central Andes of South America.

The Decade of North American Geology (DNAG) gravity grid values, spaced at 6 km, were used to produce the Gravity Anomaly Map of North America (1987; scale 1:5,000,000). Considerable caution should be exercised when using these gridded data in regions of sparse coverage (for example, deep oceanic areas). The spatial distribution of the original data from which the grid was generated is shown on sheet 5 of the published map. Note: There are no gridded data values for Mexico.

The gravity station data (1,633,499 records) were gathered by various governmental organizations (and academia) using a variety of methods. This data base was received in March 1999. Principal gravity parameters include Free-air Anomalies and Simple Bouguer Anomalies (with terrain correction applied only in areas where substantial variations in local topography exist). The gravity station data has been adjusted to the International Gravity Standardization Net 1971 (IGSN 71) and the gravity anomaly computation uses the Geodetic Reference System 1967 (GRS 67) theoretical gravity formula. The data are randomly distributed within the boundaries of the U.S., Alaska and Hawaii.

A 2.5 kilometer Bouguer anomaly grid for the state of Utah. Number of columns is 196 and number of rows is 245. The order of the data is from the lower left to the right and then up one row.

A 2 kilometer Bouguer anomaly grid for the state of Maine. Number of columns is 197 and number of rows is 292. The order of the data is from the lower left to the right and then up one row.