Mineral Resources On-Line Spatial Data
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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.
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.
This is a Raster data set. It contains the following raster data types:
The map projection used is Albers Conical Equal Area.
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.
|-99999||Areas outside the irregular survey boundary|
|Range of values|
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.
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Presentation of gridded aeromagnetic data values for use in geological and geophysical studies.
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:
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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.
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.
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.
The best resolution (spacing and flight height) flight data was used to generate the compilation. See corresponding indices in original publications for specifications.
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.
Are there legal restrictions on access or use of the data?
- Access_Constraints: None
- Use_Constraints: None
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USGS Open-File Report 97-520
The USGS has made every effort to verify the veracity of this information, but is not liable for any inaccuracies.
|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 Arc/Info export (version 7.x) Size: 8.7|
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