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 (1,522 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 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 gravity station data (24,284 records) were compiled by the U. S. Geological Survey. This data base was received on February 23, 1993. Principal gravity parameters include observed gravity, Free-air anomalies and Bouguer anomalies. 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 gravity station data (32 records) were gathered by Mr. Seth I. Gutman for AridTech Inc., Denver, Colorado using a Worden Prospector gravity meter. This data base was received in June 1985. Principal gravity parameters include Free-air Anomalies and Simple Bouguer Anomalies (no 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 area of central San Bernardino County, California.

The gravity station data (65,164 records) were gathered by various governmental organizations (and academia) using a variety of methods. The data base was received in August, 1993. Principal gravity parameters include Free-air Anomalies and Simple Bouguer Anomalies (no 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 south of 50 degrees South.

This 2' gravity density grid for the conterminous United States displays the distribution of the 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)) and by satellite altimeter-derived marine gravity anomalies computed by Sandwell and Smith (1996). The values show the number of gravity data contained in a given 2' cell. 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 gravity station data (1,037 records) were compiled by Doctor Stierman. This data base was received in June 1992. Principal gravity parameters include Free-Air Anomalies and both Simple Bouguer Anomalies (no terrain correction applied), and Complete Bouguer Anomalies (a terrain correction has been applied). The observed gravity values are referenced to the U.S.G.S. gravity base station network or to the State of Ohio gravity base station network. The 1930 International Gravity Formula was used to calculate theoretical gravity. The data are randomly distributed within north western Ohio and a portion of northwestern West Virginia.

A detailed gravimetric geoid has been computed on a 10' by 10' grid for Canada by the University of New Brunswick. This data base was received in April 1989. Principal gravity parameters include latitude, east longitude, and total geoidial height above GRS'80 reference ellipsoid. The gravity data used for the spheroidal Stokes's integration consisted of point gravity anomalies for the innermost zone, of 5'x5' mean gravity anomalies for the inner zone, and 1x1 degree mean gravity anomalies for the outer zone. The gravity values are based on the International Gravity Standardization Net 1971 (IGSN71) and the reference ellipsoid 1980 (GRS80). For detailed documentation see Technical Report No. 129.

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.