Richard W. Saltus 1997 remote-sensing image U.S. Geological Survey Open-File Report 97-520 Denver, CO U.S. Geological Survey http://pubs.usgs.gov/of/1997/ofr-97-0520/ Data from 85 individual aeromagnetic surveys were merged together to produce a single data grid spanning much of Alaska. Presentation of gridded aeromagnetic data values for use in geological and geophysical studies. 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. 1945 1982 ground condition Complete None planned -170 -130 71 54 None Aeromagnetic data None Alaska None None Robert P. Kucks U.S. Geological Survey mailing address

USGS, Box 25046, Mail Stop 964 Denver Fed Center

Denver CO 80225-0046 USA (303) 236-1405 rkucks@usgs.gov http://pubs.usgs.gov/of/1997/ofr-97-0520/gif/big_map.gif Reduced-size image of the entire state grid, 2213x1614 pixels, 451k bytes. GIF 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. At the USGS, these data are stored in several binary gridded data formats on unix systems. Data have been distributed in GEOSOFT GXF (an ASCII interchange format), Arc/Info export (e00), and ER Mapper binary formats. 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. 500 meters 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. 100 meters 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. 1945 to 1982 Patricia L. Hill U.S. Geological Survey

USGS Mail Stop 964

Denver CO 80225-0046 USA mailing address pathill@usgs.gov (303) 236-1343 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. The best resolution (spacing and flight height) flight data was used to generate the compilation. See corresponding indices in original publications for specifications. Raster Grid cell 1716 2216 Albers Conical Equal Area 55. 65. -151.0 55.0 0. 0. row and column 1000 1000 meters North American Datum of 1927 Clarke 1866 6378.20640 294.98 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. 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. -10,000 10,000 nanotesla 1-10 nanotesla -99999 Areas outside the irregular survey boundary 5.0 nanotesla Richard W. Saltus U.S. Geological Survey saltus@usgs.gov mailing address

Box 25046, MS 964 Denver Fed Center

Denver Co 80225 USA (303) 236-1375 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. Arc/Info export 7.x 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). gzip -d 8.7 http://pubs.usgs.gov/of/1997/ofr-97-0520/data/akc_e00.gz http://pubs.usgs.gov/of/1997/ofr-97-0520/data/akc_msat_e00.gz http://pubs.usgs.gov/of/1997/ofr-97-0520/data/akm_e00.gz http://pubs.usgs.gov/of/1997/ofr-97-0520/data/akm_msat_e00.gz none 20000105 Richard W. Saltus U.S. Geological Survey saltus@usgs.gov mailing address

Box 25046, MS 964 Denver Fed Center

Denver Co 80225 USA (303) 236-1375 Content Standard for Digital Geospatial Metadata FGDC-STD-001-1998