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

Frequently anticipated questions:

• What does this data set describe?
  1. How might 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 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:

   • https://pubs.usgs.gov/of/1997/ofr-97-0520/

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?

   https://pubs.usgs.gov/of/1997/ofr-97-0520/gif/big_map.gif (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

   Currentness_Reference:

   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

   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.)

   Value	Definition
   -99999	Areas outside the irregular survey boundary

   Range of values
   Minimum:	-10,000
   Maximum:	10,000
   Units:	nanotesla
   Resolution:	1-10 nanotesla

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
   

   USA

   (303) 236-1405 (voice)

   rkucks@usgs.gov

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: 1945 (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. Person who carried out this activity:
   

   Patricia L. Hill

   U.S. Geological Survey

   USGS Mail Stop 964

   Denver, CO
   

   USA

   (303) 236-1343 (voice)

   pathill@usgs.gov

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)
   
   Richard W. Saltus

   U.S. Geological Survey

   Box 25046, MS 964

   Denver Fed Center

   Denver, Co
   

   USA

   (303) 236-1375 (voice)

   saltus@usgs.gov
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?
   • Availability in digital form:

     Data format:

     Gridded files for the Alaska composite (akc*) and merged (akm*) aeromagnetic data. New versions of the grids were added to the web site in February 1999. These grids are akc_msat* and akm_msat*. The new grids contain a regional surface correction based on a satellite magnetic model of the long wavelengths of the Earth's magnetic field (see March 1999 issue of GSA Today for more information). The original grids contained a questionable long-wavelength trend which caused the NW portion of the grids to be tipped downward (there was also a spurious trend with a different slope in SE Alaska). in format ARCE (version 7.x) Size: 8.7

     Network links:

     https://pubs.usgs.gov/of/1997/ofr-97-0520/data/akc_e00.gz https://pubs.usgs.gov/of/1997/ofr-97-0520/data/akc_msat_e00.gz https://pubs.usgs.gov/of/1997/ofr-97-0520/data/akm_e00.gz https://pubs.usgs.gov/of/1997/ofr-97-0520/data/akm_msat_e00.gz

   • Cost to order the data: none

Who wrote the metadata?

Dates:

Last modified: 09-Feb-2016

Metadata author:

Richard W. Saltus

U.S. Geological Survey

Box 25046, MS 964

Denver Fed Center

Denver, Co

USA

(303) 236-1375 (voice)

saltus@usgs.gov

Metadata standard:

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