Composite and Merged Aeromagnetic Data for Alaska - A Web Site for Distribution of Gridded Data and Plot Files

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Frequently anticipated questions:

What does this data set describe?

Composite and Merged Aeromagnetic Data for Alaska - A Web Site for Distribution of Gridded Data and Plot Files
Data from 85 individual aeromagnetic surveys were merged together to produce a single data grid spanning much of Alaska.
This file defines the background of the grid that was used to create the image displayed on the website. The geo-tiff image with a subtle light/dark color scale was created using the commercial software package Geosoft. The resolution is 200 dpi.

Note that grid origin is at the center of the SW grid cell (first value in the grid file). The location of this point is at X=-902,000 and Y=110,000 meters relative to the projection described below.
  1. How might this data set be cited?
    Saltus, Richard W., 1997, Composite and Merged Aeromagnetic Data for Alaska - A Web Site for Distribution of Gridded Data and Plot Files: U.S. Geological Survey Open-File Report 97-520, U.S. Geological Survey, Denver, CO.

    Online Links:

  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -170
    East_Bounding_Coordinate: -130
    North_Bounding_Coordinate: 71
    South_Bounding_Coordinate: 54
  3. What does it look like? (GIF)
    Reduced-size image of the entire state grid, 2213x1614 pixels, 451k bytes.
  4. Does the data set describe conditions during a particular time period?
    Beginning_Date: 1945
    Ending_Date: 1982
    ground condition
  5. What is the general form of this data set?
    Geospatial_Data_Presentation_Form: remote-sensing image
  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 1716 x 2216, 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: 55.
      Standard_Parallel: 65.
      Longitude_of_Central_Meridian: -151.0
      Latitude_of_Projection_Origin: 55.0
      False_Easting: 0.
      False_Northing: 0.
      Planar coordinates are encoded using row and column
      Abscissae (x-coordinates) are specified to the nearest 1000
      Ordinates (y-coordinates) are specified to the nearest 1000
      Planar coordinates are specified in meters
      The horizontal datum used is North American Datum of 1927.
      The ellipsoid used is Clarke 1866.
      The semi-major axis of the ellipsoid used is 6378.20640.
      The flattening of the ellipsoid used is 1/294.98.
  7. How does the data set describe geographic features?
    Grid cell
    Area of the earth's surface represented by an observation (Source: this dataset)
    Residual total intensity of Earth's magnetic field
    Value, in nanotesla, of total intensity of Earth's magnetic field after removal of the theoretical International Geomagnetic Reference Field value. (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.)
    -99999Areas outside the irregular survey boundary
    Range of values

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)
    • Richard W. Saltus
  2. Who also contributed to the data set?
    Data were collected between 1945 and 1982 by the U.S. Geological Survey and by private contractors under contract to the U.S. Geological Survey and other government organizations.
  3. To whom should users address questions about the data?
    Robert P. Kucks
    U.S. Geological Survey
    USGS, Box 25046, Mail Stop 964
    Denver Fed Center
    Denver, CO

    (303) 236-1405 (voice)

Why was the data set created?

Presentation of gridded aeromagnetic data values for use in geological and geophysical studies.

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: 1982 (process 1 of 1)
    Collection of aeromagnetic data.

    The "state-of-the-art" for aeromagnetic data collection changed significantly throughout the data collection period for this data set. Journals such as GEOPHYSICS have periodic articles describing the contemporaneous state of standard procedures. From 1945 to 1982. Person who carried out this activity:
    Patricia L. Hill
    U.S. Geological Survey
    USGS Mail Stop 964
    Denver, CO

    (303) 236-1343 (voice)
  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?
    Grid values represent the total intensity of the Earth's magnetic field after removal of the International Geomagnetic Reference Field (IGRF). Measurements were made by a variety of magnetometer systems with typical accuracy of 1 to 10 nanotesla (nT).

    In addition to corrections for the IGRF, the individual magnetic surveys typically contain corrections for the diurnal variations of the Earth's magnetic field that occurred during the flight (as recorded by a base station magnetometer) and leveling adjustments made at flightline intersections. The absolute accuracy of the gridded data are probably better than 5 nanotesla, with accuracy increasing for with increasing wavelength.
  2. How accurate are the geographic locations?
    Horizontal position of the aircraft were determined by various means. Older surveys (1945-1960s) were positioned by reconciling down-looking photographs with topographic maps and orthophoto quad maps. Later surveys used various standard aircraft navigation aids as they became available.
  3. How accurate are the heights or depths?
    The individual aeromagnetic surveys that make up this data set were flown at various fixed barometric altitudes and as draped surveys with constant terrain clearance specifications.
  4. Where are the gaps in the data? What is missing?
    The best resolution (spacing and flight height) flight data was used to generate the compilation. See corresponding indices in original publications for specifications.
  5. How consistent are the relationships among the observations, including topology?
    The flight data collected to generate the final gridded product is dependent on the individual contractor. All corresponding corrections were then made by the contractor and a final data set was presented for gridding. Compositing of the individual surveys was done in an analog process before digitizing. The process of reprojecting the grid does not affect the accuracy of the product.

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)
    Peter N Schweitzer
    USGS Geology, Energy, and Minerals Science Center
    12201 Sunrise Valley Drive
    Reston, VA

    703-648-6533 (voice)
    703-648-6252 (FAX)
  2. What's the catalog number I need to order this data set? USGS Open-File Report 97-520
  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?

Last modified: 04-Aug-2021
Metadata author:
Peter N Schweitzer
USGS Geology, Energy, and Minerals Science Center
12201 Sunrise Valley Drive
Reston, VA

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

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