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Bouguer gravity anomaly data grid for the conterminous US

Frequently-anticipated questions:

• What does this data set describe?
  1. How should this data set be cited?
  2. What geographic area does the data set cover?
  3. What does it look like?
  4. Does the data set describe conditions during a particular time period?
  5. What is the general form of this data set?
  6. How does the data set represent geographic features?
  7. How does the data set describe geographic features?
• Who produced the data set?
  1. Who are the originators of the data set?
  2. Who also contributed to the data set?
  3. To whom should users address questions about the data?
• Why was the data set created?
• How was the data set created?
  1. From what previous works were the data drawn?
  2. How were the data generated, processed, and modified?
  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?
  2. How accurate are the geographic locations?
  3. How accurate are the heights or depths?
  4. Where are the gaps in the data? What is missing?
  5. How consistent are the relationships among the data, including topology?
• How can someone get a copy of the data set?
  1. Are there legal restrictions on access or use of the data?
  2. Who distributes the data?
  3. What's the catalog number I need to order this data set?
  4. What legal disclaimers am I supposed to read?
  5. How can I download or order the data?
• Who wrote the metadata?

What does this data set describe?

1. How should this data set be cited?

   Kucks, Robert P. , 1999, Bouguer gravity anomaly data grid for the conterminous US:.

   This is part of the following larger work.

   U.S. Geological Survey, 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.

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.)

   Value	Definition
   -0.3402823E+39	areas 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 80225-0046
   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 80225-0046
   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 80225
   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?
   • Availability in digital form:

     Data format:	Gridded gravity anomaly data in format ArcInfo export (version 7.x) Arc-Info grid format Size: 3.5 megabytes
     Network links:	<ftp://ftpext.usgs.gov/pub/cr/co/denver/musette/pub/GEOPHYSICAL_DATA/cdrom_DDS-9/GRIDS/ARC_INFO/bouguer_sdd.e00.gz>

   • Cost to order the data: none

Who wrote the metadata?

Dates:

Last modified: 27-Feb-2003

Metadata author:

Robert P. Kucks
U.S. Geological Survey
Box 25046, MS 964
Denver Federal Center
Denver, CO 80225
USA

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

Metadata standard:

Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)