Robert P. Kucks 1999 U.S. Geological Survey 1993 U.S. Geological Survey Digital Data Series DDS-9 The grid of gravity anomaly data for the conterminous United States and adjacent marine areas was constructed from National Information Mapping Agency (NIMA) gravity data files. It was reprojected to the specifications for the Spatial Data Delivery (SDD) project. Digital Data Series DDS-9 (CD-ROM) contains gridded geophysical data (gravity anomaly data only defined here) images generated from the data, software for displaying and analyzing the images, software for processing potential-field geophysical data, and documentation files that describe the data and software. Gravity anomalies are produced by density variations within the rocks of the Earth's crust and upper mantle. Mapping of these density variations is the primary use of gravity anomalies. 1998 1999 reprocessing for SDD website Complete None planned -125 -65 49 25 None None Robert P. Kucks U.S. Geological Survey mailing address

Box 25046, MS 964 Denver Federal Center

Denver CO 80225-0046 USA (303) 236-1405 rkucks@usgs.gov The compilation of onshore data consisted of nearly one million Bouguer gravity anomaly values computed by using a reduction density of 2.67 grams per cubic centimeter. The offshore data consisted of approximately 800,000 free-air gravity anomaly values. USGS employees: Godson and Scheibe At the USGS, these data are stored in several binary gridded data formats on UNIX and PC systems. Data have been distributed in GEOSOFT GXF (an ASCII interchange format), Arc/Info export (e00), and ER Mapper binary format. none gravity, Bouguer none United States Gravity measurements made on the surface of the Earth must be corrected in various way before they can be made into an anomaly map. The free-air correction reduces the measurement to sea level by assuming there is no intervening mass as a uniform slab of constant density, and the complete Bouguer correction includes the effects of constant density topography within 166.7 km of the measurement location. A gravity reference field is subtracted from the corrected measurements to produce the free-air, simple Bouguer, or complete Bouguer anomaly. The data set constitutes many years of individual collection of gravity stations and therefore is difficult to access the precise accuracy. overall station accuracy of less than 2 milligals Earlier collection of stations was by surveying methods. Positions were also acquired from locations on topographic maps. Recently the station latitudes, longitudes and elevations were acquired by Geopositioning Satellite (GPS) systems. 100 surveying methods - 1.0 meter topographic maps - 3.0 meter GPS systems - 100.0 meter/1.0 meter differential 100 surveying methods - 1.0 meter topographic maps - 1.0-20.0 meter GPS systems - 100 meter/1.0 meter differential gravity anomaly grid was extracted from DDS-9 and reprojected using USGS in-house developed software. In-house software was also used to convert from USGS grid format to Arc-Info grid format. The "state-of-the-art" for gravity data collection changed significantly throughout the data collection period for this data set. Journals such as GEOPHYSICS have periodic articles describing the current state of standard procedures. 1999 Robert P. Kucks U.S. Geological Survey

Box 25046, MS 964 Denver Federal Center

Denver CO 80225-0046 USA mailing address rkucks@usgs.gov (303)236-1405 The accuracy of the data set that generated the grid is totally dependant on the individual who collected each gravity station. In the case of a national set they would number in the hundreds. Variations would include latitude, longitude, elevation, raw gravity reading and base station control. There will always be upgrades and additions to the data set that generated the grid due to regional interests. Raster Grid cell 776 1321 29.5 45.5 -96.0 37.5 0. 0. Albers Conical Equal Area row and column 4.0 4.0 kilometer Noth American Datum of 1927 CLARKE1866 6378.20640 0.99661 grid cell Area of Earth's surface for which gravity anomaly is summarized cell value Value, in milligals, of the gravity anomaly of Earth's gravitational field. Paterson, N.R., and Reeves, C.V., 1985, Applications of gravity and magnetic surveys: The state-of-the-art in 1985: Geophysics, v. 50, p. 2558-2594. -340 95 milligal 0.1 -0.3402823E+39 areas for which no data are available in the grid Robert P. Kucks U.S. Geological Survey mailing address

Box 25046, MS 964 Denver Federal Center

Denver CO 80225 USA (303)236-1405 rkucks@usgs.gov USGS Digital Data Series DDS-9 The USGS has made every effort to verify the veracity of this information, but is not liable for any inaccuracies. ArcInfo export 7.x Arc-Info grid format Gridded gravity anomaly data gunzip 3.5 megabytes ftp://ftpext.usgs.gov/pub/cr/co/denver/musette/pub/GEOPHYSICAL_DATA/cdrom_DDS-9/GRIDS/ARC_INFO/bouguer_sdd.e00.gz none 20030227 Robert P. Kucks U.S. Geological Survey mailing address

Box 25046, MS 964 Denver Federal Center

Denver CO 80225 USA (303)236-1405 rkucks@usgs.gov Content Standard for Digital Geospatial Metadata FGDC-STD-001-1998