This 2' surface deflection of the vertical grid for Mexico, and North-Central is the DMEX97 model. The computation used about one million terrestrial and marine gravity measurements held in the NGS database as of March 1997. These gravity measurements were augmented by data contributions from NGA (former National Imagery and Mapping Agency), and satellitealtimeter derived gravity anomalies computed by Sandwell and Smith (1997). The deflection values themselves were obtained by numericaldifferentiation of cubic spline models along the parallels of the MEXICO97 geoid grid, followed by a correction for curvature of the plumb line, yielding surface deflections of the vertical. Since the EGM96 model was used to fill data gaps south of 20 degrees North latitude, the deflections in and around those regions will be less accurate. The gravity values are based on the International Gravity Standardization Net 1971 (IGSN71). The deflections are referred to Geodetic Reference System 1980 (GRS80) ellipsoidalnormals oriented in the International Terrestrial Reference Frame 1994 (ITRF94(1996.0)). 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 (21,242 records) were compiled by the University of Texas at El Paso. This data base was received in October 1984. 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 boundaries of the State of New Mexico.

This 2' geoid height grid for Mexico, and North-Central America, is the MEXICO97 geoid model. The computation used about one million terrestrial and marine gravity measurements held in the NGS database as of March 1997. These gravity measurements were augmented by data contributions from NGA (former National Imagery and Mapping Agency), and satellite altimeter-derived gravity anomalies computed by Sandwell and Smith (1997). Large data gaps south of 20 degrees North latitude were filled with 15'x15' gravity values derived from the EGM96 global geopotential model. This helped control interpolation errors across the data gaps during the gridding of terrain corrected Bouguer anomalies. After gridding of the Bouguer anomalies, the Bouguer plate was restored, a degree 360 model of gravity anomalies (from EGM96) was removed, and the residual free-air anomalies in the data gaps (south of 20 degrees North Latitude) were zeroed. (This zeroing was found to be the best way currently available to yield a reasonable geoid in the data gaps). The residual Faye anomalies were converted to residual co-geoid undulations through a 1-D FFT formulation of Stokes' integral, and finally the EGM96 undulation model was restored, and the indirect effect applied. This means that in the data gaps, long wavelength information is provided by EGM96, short wavelength information is provided by the 2'x2' DTED (during the application of the indirect effect), but medium wavelength information (usually provided by gravity measurements) is missing altogether. Although the exact accuracy of the geoid in the data gaps is difficult to ascertain, the quoted accuracy for EGM96 (which is the primary source of geoid information in the data gaps) is below 50 cm in these areas. 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, centered at the origin of the International Terrestrial Reference Frame 1994 (ITRF94(1996.0)). Additional information is available at: http://www.ngs.noaa.gov/GEOID/MEXICO97/ We are particularly grateful to NGA (former National Imagery and Mapping Agency) for their assistance and their data contributions.

This 2' surface deflection of the vertical grid for the conterminous United States is the DEFLEC96 model. The computationused 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)). The deflections were obtained by numerical differentiation of cubic spline models along the meridians of the GEOID96 grid. The gravity values are based on theInternational Gravity Standardization Net 1971 (IGSN71). The deflections are referred to the Geodetic Reference System 1980 (GRS80) ellipsoidal normals. The deflections of the vertical incorporate the reference system relationship between NAD 83(86) and ITRF94(1996.0), the datum offset of NAVD 88, and thecontributions from 2951 GPS on leveled benchmarks. The curvature of the plumb line correction has been applied. http://www.ngs.noaa.gov/DEFLEC/deflec.html We are particularly grateful to NGA (former National Imagery and Mapping Agency) for their assistance and their data contributions.

The gravity station data (152,624 records) were compiled by the University of Texas at Dallas. This data base was received in June 1992. 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 boundaries and offshore of South America.

This 2' surface deflection of the vertical grid for Puerto Rico and the Virgin Islands is distributed as the DEFLEC96 model. The computation used about 26,000 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)). The deflections were obtained by numerical differentiation of cubic spline models along the meridians of the GEOID96 grid. The gravity values are based on the International Gravity Standardization Net 1971IGSN71). The deflections are referred to theGeodetic Reference System 1980 (GRS80) ellipsoidal normals. The curvature of the plumbline correction has been applied. We are particularly grateful to NGA (former National Imagery and Mapping Agency) for theirassistance and their data contributions.

This 2' surface deflection of the vertical grid for the conterminous United States is the DEFLEC96 model. The computationused about 1.8 million terrestrial and marine gravity data held in the National Geodetic Survey gravity data base in July 1996. Thesedata were augmented by gravity data contributions from NGA (former National Imagery and Mapping Agency (former Defence Mapping Agency)). The deflections were obtained by numerical differentiation of cubic spline models along theparallels of the GEOID96 grid. The gravity values are based on the International Gravity Standardization Net 1971 (IGSN71). The deflections are referred to the Geodetic Reference System 1980 GRS80) ellipsoidal normals. The deflections of the verticalincorporate 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. The curvatureof the plumb line correction has been applied. We are particularly grateful to NGA (former National Imagery and Mapping Agency) for their assistance and their data contributions.

This 2' x 4' surface deflection of the vertical grid for Alaska is the DEFLEC96 model. The computation used about 1.1 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 DefenseMapping Agency). The deflections were obtained by numerical differentiation of cubic spline models along the meridians of the GEOID96 grid. The gravity values are based on the International Gravity Standardization Net 1971 (IGSN71). The deflections are referred to the Geodetic Reference System 1980 (GRS80) ellipsoidal normals. The curvature of the plumb line correction has been applied.Additional information is available at:http://www.ngs.noaa.gov/ We are particularlygrateful to NGA (former National Imagery and Mapping Agency) for their assistance and their data contributions.

This 2' gravity density grid for Mexico, North-Central America, and the Western Caribbean Sea shows the distribution of about one million terrestrial and marine gravity measurements held in the NGS database as of March 1997. These gravity measurements have been augmented by data contributions from NGA (former National Imagery and Mapping Agency), and satellite altimeter-derived gravity anomalies computed by Sandwell and Smith (1997). This density grid shows the number of gravity data contained in a 2' x 2' cell. Please note that this coverage grid does not show the 15'x15' EGM96 gravity values used to fill the void areas south of 20 degrees North. Additional information is available at: http://www.ngs.noaa.gov/GEOID/MEXICO97 .We are particularly grateful to NGA (former National Imagery and Mapping Agency) for their assistance and their data contributions.

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.