Contributing Organization: Geological Society of America
Summary: 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.
Additional information is available.
Note: There are no gridded data values for Mexico. All data were transformed to map coordinates using the spherical North American transverse Mercator projection with a central meridian of longitude 100 degrees W, a scale factor of 0.926, and a radius of 6371.204 km. The forward (STMFWD) and inverse (STMINV) projection routines, which are included with this documentation, can be used with this data.
The user should use CAUTION, however, when applying this code in two specific cases:
The data were compiled from about 2 million gravity anomaly values derived from surface, airborne, and satellite measurements. For continental areas, anomalies have been computed from measurements made with static gravimeters operated on dry land, sea and lake bottoms, and the frozen surfaces of the sea or lakes. Over oceanic areas, data were acquired with dynamic gravimeters operated on surface ships or in helicopters.
For Alaska, Chukchi Sea, Western Beaufort Sea, Norton Sound,Caribbean Islands, land and offshore Colombia, eastern Panama, parts of southern Mexico, and the Mexico-Panama Pacific margin, where surface data could not be readily obtained in digital form, contour maps provided synthetic point gravity values.
Over the oceans, up to about 70 degrees North latitude, satellite-derived free-air values filled in areas where surface data were either unavailable or too sparse to provide reasonable definition of the gravity field.
Computation of the gravity anomalies was based on the International Gravity Standardization Net 1971 and the Geodetic Reference System 1967. Bouguer gravity anomalies were calculated using a standard crustal density of 2670 kg/m3. The data for Venezuela and the high-relief areas of Canada and the United States have been terrain corrected. Elsewhere, the data are generally not terrain corrected. The estimated rms error for land, surface marine, and airborne data ranges from +/- 1 to +/- 5 milligals. Satellite data have an estimated rms error of +/- 8 milligals.
Data were interpolated to a uniform grid using the minimum curvature method. Satellite data, which were provided on a 15- minute geographic grid, were first gridded at a 24-km interval to remove the short-wavelength components at higher latitudes and then combined with the surface data. The resultant data set was then interpolated to a 6-km grid. Data sources and the individuals or agencies responsible for the compilation and integrity of each source are listed below. Where data from two or more sources overlap, the data set with the highest integrity has been used. The final selection of data from each source is shown by the data distribution index map which accompanies the Gravity Anomaly Map of North America.
DNAG Gravity Data Sources and References
Bering Sea/Marine: Childs, J.R., Magistrale, H.W., and Cooper, A.K., 1985, Free-air gravity anomaly map of the Bering Sea: U.S. Geological Survey Miscellaneous Field Studies Map MF-1728, scale 1:2,500,000.
Alaska/Marine: Fisher, M.A., Childs, J.R., and Magistrale, H.W., 1982, Free-air gravity map, Norton Basin, Alaska: U.S. Geological Survey, Miscellaneous Field Studies Map MF-1460, scale 1:250,000.
Alaska/Land: Barnes, D.F., 1977, Bouguer gravity map of Alaska: U.S. Geological Survey Geophysical Investigations Map GP-913, scale 1:2,500,000.
Chukchi and Beaufort Seas/Marine: May, S.D., 1985, Free-air gravity anomaly map of the Chukchi and Alaskan Beaufort seas, Arctic Ocean: U.S. Geological Survey Miscellaneous Investigations Series Map I-1182E, scale 1:1,000,000.
Greenland/Land and Marine: Anderson, O.B., 1973, Surface-ship gravity measurements in the Davis Strait, western Greenland: Geodaetisk Instituts Skrifter, 3. RK., v. 39.
Blundell, D.J., 1978, A gravity survey across the Gardar Igneous Province, southwest Greenland: Journal of the Geological Society of London, v. 135, pp. 545-554.
Bull, C., 1955, Values of gravity on the inland ice in north Greenland: Meddelelser om Gronland, v. 137, no. 1, part 3, 11 pp.
Forsberg, R., 1986, Gravity measurements in Jameson Land and neighboring parts of east Greenland: Meddelelser om Gronland, Geoscience, no. 15, 23 pp.
Hoisl, R., 1965, Gravimetermessungen uber das Gronlandisches Inlandseis auf einer West-Ost profil (E.G.I.G.) 1959: Deutsche Geodaetische Kommission, Reihe C, Heft 85, 53 pp.
Kejlso, E., 1958, Gravity measurements in western Greenland 1950- 1952: Geodaetisk Instituts Skrifter, 3. RK., v. 27, 69 pp.
Saxov, S.E., 1958, Gravity in western Greenland: Geodaetisk Instituts Skrifter, 3. RK., v. 29.
Svejgaard, B., 1959, Gravity measurements in western Greenland, 1953-1955: Geodaetisk Instituts Skrifter, 3. RK., v. 32, 19 pp.
Digital data were provided by the Danish Geodetic Institute, Gamlehave Alle 22, DK-2920, Charlottenlund, Denmark.
Canadian Land and Offshore/Land and Marine: Digital data from the National Gravity Data Base were provided by the Geophysical Data Centre, Geophysics Division, Geological Survey of Canada, Ottawa, Canada.
Continental U.S./Land: Godson, R.H., and Scheibe, D.M., 1982, Description of magnetic tape containing conterminous U.S. gravity data in a gridded format: U.S. Department of Commerce National Technical Information Service, PB82-254798.
Digital data were provided by U.S. Geological Survey, Denver, Colorado.
Northern Mexico/Land: Digital data were provided by C.L.V. Aiken, Center for Lithospheric Studies, University of Texas at Dallas, and M.F. de la Fuente Duch, Geosciencias Applicadas SA, Mexico, D.F.
Southern Mexico/Land: Digital data were provided by M.F. de la Fuente, Geosciencias Applicadas SA, Mexico, D.F., Carlos L.V. Aiken, Center for Lithospheric Studies, University of Texas at Dallas, and M. Mena Jara, Instituto de Geofisica, Universidad Nacional Autonoma, Mexico D.F.
Central America/Land: Digital data were provided by C.L.V. Aiken, V.M. Godley and J.N. Kellogg, Hawaii Institute of Geophysics, Honolulu, Hawaii, and M. Zuniga, Universidad Nacional Autonoma de Honduras.
Central and South America/Land and Marine: Kellogg, J.N., Godley, V.M., Woollard, G.P., Ropain, C., and Bermudez Gomez, A., 1983, Simple Bouguer gravity map of Columbia, eastern Panama, and adjacent marine areas: Transactions of the 10th Caribbean Geological Conference, Cartegena, Columbia.
Kellogg, J.N., Godley, V.M., Woollard, G.P., Ropain, C., Bermudez Gomez, A., and Murphy, T., Free-air gravity map of Columbia, eastern Panama, and adjacent marine areas: submitted to Geological Society of America Map and Chart Series.
Digital data were provided by J.N. Kellogg, formerly of Hawaii Institute of Geophysics (HIG), Honolulu, Hawaii and presently at University of South Carolina, Columbia, South Carolina, and V.M. Godley, HIG, Honolulu, Hawaii.
Venezuela/Land and Marine: Digital data were provided by V. Graterol from the Universidad Simon Bolivar (USB) gravity data base, including the following data sets:
The Caribbean region: Boulder, Colorado, Geological Society of America, The Geology of North America, v. H, scale: 1:5,000,000.
Bahamas/Land and Marine (via Helicopter): Helicopter gravity survey flown by Carson Geoscience Company, 1985. Digital data were provided by U.S. Department of Defense Gravity Library.
Pacific Margin and Gulf of Mexico/Marine: Couch, R.W., Ness, G.E., Victor, L., Shanahan, S., and Troseth, S.C., 1986, Free-air gravity anomalies, Southern Mexico to Costa Rica margin in Ladd, J.W., and Buffler, R.T., eds., Middle America Trench off Western America: Marine Science International, Ocean Drilling Program, Regional Atlas Series.
Digital data were provided by R.W. Couch and G.E. Ness, Oregon State University, Corvallis, Oregon, for Gulf of California and offshore Yucatan. Other digital data from R.W. Couch and G.E. Ness for the margins of Oregon, Washington, British Columbia, Alaska and Mexico are included in the U.S. Department of Defense Gravity Library data.
Atlantic, Pacific and Arctic Oceans and Caribbean Sea/Marine: U. S. Department of Defense Gravity Library, St. Louis, Missouri.
Open Oceans/(via Satellite): Balmino, G., Moynot, B., Sarrailh, M., and Vales, N., 1987, Free- air gravity anomalies over the oceans from Seasat and GEOS 3 altimeter data: EOS, v. 68, no. 2, pp. 17-18.