U.S. DEPARTMENT OF THE INTERIOR

                          U.S. Geological Survey









Digital Aeromagnetic Datasets for the Conterminous United States and Hawaii
                                     -


          A Companion to the North American Magnetic Anomaly Map


                                Version 1.0







                                    By

                          U.S. Geological Survey
                                  [Image]

                                    and


                     National Geophysical Data Center
                                  [Image]




                          Open-File-Report 02-361



                                    2002

README.TXT


About This DVD

After pioneering the first airborne magnetic survey in 1945, the U.S.
Geological Survey  (USGS) has collected piecemeal aeromagnetic data for
most of the U.S., including offshore areas on both coasts. The U.S.
Geological Survey's digital and analog archives comprise more than 1,000
surveys, covering approximately 8,000,000 line-km of data, flown at various
flight heights and line spacings.  The purpose of this report is to compile
and standardize original aeromagnetic survey data (approximately 5,800,00
line-km) and make them available to the public.

This DVD contains currently available digital magnetic files for the
conterminous U.S. and Hawaii from the USGS archives.  Updates and
corrections to files included here can be found on the online release of
the datasets (http://pubs.usgs.gov/of/2002/ofr-02-361).

Data Format for ASCII .XYZ Files

Format
FORTRAN ASCII data formats specified as follows:  I = integer, F = floating
point.  Column widths given as total width and number of decimal points

Example of .XYZ file format

    2 270   -89.2373  41.5869 1981 100   1520   994.1  1791.3 57483.93
4.96
    2 270   -89.2380  41.5869 1981 100   1530   984.3  1791.3 57483.18
4.10
    2 270   -89.2386  41.5869 1981 100   1540   984.3  1791.3 57483.43
4.37
    2 270   -89.2392  41.5869 1981 100   1550   987.5  1768.4 57481.93
2.91

Line_no (I5)
Directn (I4) - flight line direction, azimuth degrees clockwise from north
Longitude (F11.4) - decimal degrees
Latitude (F9.4) - decimal degrees
Year (I5)
Jul_day (I4) - Julian day
Fiducial (I7)
Radar (F8.1) - radar altimeter reading, in feet
Barom (F8.1) - barometric altimeter reading, in feet
Totmag (F9.2) - total magnetic value, diurnally corrected, in nT
Resmag (F9.2) - total magnetic value minus a Geomagnetic Reference Field
(GRF), in nT
1X

NOTE: longitude and latitude are defined to four decimal places (10 meter
resolution) because this is the most reasonable accuracy that can be
standardized for the variety and scaling of many of the original maps.

A Geosoft import template (import_template.i3) is available on the ftp site
(ftp://musette.cr.usgs.gov/pub/NA_MAGNETICS/digital_data).

Processing Specifications (.XYZ and .JPG)

Digital databases were imported, edited in a cursory fashion, and exported
using Geosoft Oasis montaj software, version 4.3 and higher.  Errors may
still remain.  A standardized template was created (format above) from
which all .XYZ files were exported.  *.JPG images of the data were created
using a Geographic Coordinate (longitude, latitude) System and the Clarke
1866 datum.  Specific processing information, including types and extent of
editing and processing, errors still present in data, application of
Geomagnetic Reference Field (GRF), are given in the corresponding metadata
file for each survey.


Format for Metadata Files

The format for metadata files is based on the Federal Geographic Data
Committee (FGDC) Content Standards for Digital Geospatial Metadata.
Metadata files are also included at the National Geospatial Data
Clearinghouse, but repeated here for convenience and association with
survey .XYZ files and .jpg images.


How to Run This DVD

   * Load the DVD and read the readme file. Change the directory to E:\ (E
     is assumed to be the users DVD reader).

   * If you are running this DVD on a PC using Windows 95, 98, 2000, NT,
     ME, or XP or on a Macintosh, simply double click on the file named
     "mag_home.htm".

   * For UNIX and Windows 3.x systems, load your browser into memory and
     then execute the "mag_home.htm" file.

NOTE: This DVD was developed on a Pentium III PC using the Hyper Text
Markup Language.  In theory because this is an HTML product, it should run
on most platforms using various browsers.


If you have any questions about or problems with the data on this DVD,
please contact Sarah Shearer or Pat Hill.

* This report is preliminary and has not been reviewed for conformity with
U.S. Geological Survey editorial standards. Use of brand names is for
descriptive purposes and does not constitute endorsement by the U.S.
Geological Survey.