Bouguer gravity anomaly data grid for the conterminous US

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Frequently anticipated questions:


What does this data set describe?

Title: Bouguer gravity anomaly data grid for the conterminous US
Abstract:
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.
Supplemental_Information:
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.
  1. How might this data set be cited?
    Kucks, Robert P., 1999, Bouguer gravity anomaly data grid for the conterminous US:.

    Online Links:

    This is part of the following larger work.

    Phillips, Jeffrey D., Duval, Joseph S., and Ambroziak, Russell A., 1993, National geophysical data grids; gamma-ray, gravity, magnetic and topographic data for the conterminous United States: U.S. Geological Survey Digital Data Series DDS-9.

    Online Links:

  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -125
    East_Bounding_Coordinate: -65
    North_Bounding_Coordinate: 49
    South_Bounding_Coordinate: 25
  3. What does it look like?
  4. Does the data set describe conditions during a particular time period?
    Beginning_Date: 1998
    Ending_Date: 1999
    Currentness_Reference:
    reprocessing for SDD website
  5. What is the general form of this data set?
  6. How does the data set represent geographic features?
    1. How are geographic features stored in the data set?
      This is a Raster data set. It contains the following raster data types:
      • Dimensions 776 x 1321, type Grid cell
    2. What coordinate system is used to represent geographic features?
      The map projection used is Albers Conical Equal Area.
      Projection parameters:
      Standard_Parallel: 29.5
      Standard_Parallel: 45.5
      Longitude_of_Central_Meridian: -96.0
      Latitude_of_Projection_Origin: 37.5
      False_Easting: 0.
      False_Northing: 0.
      Planar coordinates are encoded using row and column
      Abscissae (x-coordinates) are specified to the nearest 4.0
      Ordinates (y-coordinates) are specified to the nearest 4.0
      Planar coordinates are specified in kilometer
      The horizontal datum used is Noth American Datum of 1927.
      The ellipsoid used is CLARKE1866.
      The semi-major axis of the ellipsoid used is 6378.20640.
      The flattening of the ellipsoid used is 1/0.99661.
  7. How does the data set describe geographic features?
    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. (Source: 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.)
    ValueDefinition
    -0.3402823E+39areas for which no data are available in the grid
    Range of values
    Minimum:-340
    Maximum:95
    Units:milligal
    Resolution:0.1

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)
    • Robert P. Kucks
  2. Who also contributed to the data set?
    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
  3. To whom should users address questions about the data?
    Robert P. Kucks
    U.S. Geological Survey
    Box 25046, MS 964
    Denver Federal Center
    Denver, CO
    USA

    (303) 236-1405 (voice)
    rkucks@usgs.gov

Why was the data set created?

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.

How was the data set created?

  1. From what previous works were the data drawn?
  2. How were the data generated, processed, and modified?
    Date: 1999 (process 1 of 1)
    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. Person who carried out this activity:
    Robert P. Kucks
    U.S. Geological Survey
    Box 25046, MS 964
    Denver Federal Center
    Denver, CO
    USA

    (303)236-1405 (voice)
    rkucks@usgs.gov
    Data sources used in this process:
    • 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.
  3. What similar or related data should the user be aware of?

How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?
    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.
  2. How accurate are the geographic locations?
    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.
  3. How accurate are the heights or depths?
  4. Where are the gaps in the data? What is missing?
    There will always be upgrades and additions to the data set that generated the grid due to regional interests.
  5. How consistent are the relationships among the observations, including topology?
    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.

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?
Access_Constraints: None
Use_Constraints: None
  1. Who distributes the data set? (Distributor 1 of 1)
    Robert P. Kucks
    U.S. Geological Survey
    Box 25046, MS 964
    Denver Federal Center
    Denver, CO
    USA

    (303)236-1405 (voice)
    rkucks@usgs.gov
  2. What's the catalog number I need to order this data set? USGS Digital Data Series DDS-9
  3. What legal disclaimers am I supposed to read?
    The USGS has made every effort to verify the veracity of this information, but is not liable for any inaccuracies.
  4. How can I download or order the data?

Who wrote the metadata?

Dates:
Last modified: 07-Mar-2018
Metadata author:
Peter N Schweitzer
USGS Eastern Mineral and Environmental Resources Science Center
Geologist
12201 Sunrise Valley Drive
Reston, VA
USA

703-648-6533 (voice)
703-648-6252 (FAX)
pschweitzer@usgs.gov
Metadata standard:
Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)

This page is <https://mrdata.usgs.gov/metadata/usgravboug.faq.html>
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