Digital Versions of the Amargosa Aeromagnetic Survey
----------------------------------------------------

The following describes digital versions of the Amargosa Desert (California and 
Nevada) aeromagnetic survey available for downloading from a U.S. Geological 
Survey anonymous FTP directory.  The aeromagnetic survey was conducted in 1999 
by Sander Geophysics Limited under contract to the USGS.  For additional 
information about the data acquisition, processing, and interpretation, please 
refer to U.S. Geological Survey Open-File Report 00-188, also available on the 
Internet at http://geopubs.wr.usgs.gov/open-file/of00-188. 

The digital data are available at anonymous ftp site, 

   ftp://geopubs.wr.usgs.gov/pub/open-file/of00-188/data.

Two ascii formats are available:

amargosa.xyz.gz
---------------
This file provides discrete point measurements along each flight line.  Flight 
lines are highly oversampled, and only every tenth datum is included in this 
file.  The format description is as follows:
 
Channel                 Columns   Contents             Format
-------                 -------   --------             ------
Line identifier          01-08    llllssqq (see below)  a8
Longitude                09-20    NAD-27, decimal deg   f12.6
Latitude                 21-30    NAD-27, decimal deg   f10.6
Radar height             31-37    meters                f7.1
Barometric height        38-44    meters                f7.1
Uncorr. magnetic field   45-53    nT                    f9.2
Total-field anomaly      54-62    nT                    f9.2
Levelled anomaly         63-71    nT                    f9.2

Line identifier consists of line number, segment number, and quadrant.

amargosa.gxf.gz
---------------
This file is a grid of total-field-anomaly values interpolated from point 
measurements.  The grid was computed by transforming flight-line point data to 
Universal Transverse Mercator projection (NAD-27, Zone 11N), then using a 
minimum-curvature algorithm to interpolate values to rectangular grid 
intersections.  Grid intersections are spaced 100 m apart.  The file is in 
Geosoft Grid eXchange Format (GXF), a standard ASCII format for exchanging 
gridded data among different software systems.  GXF has been adopted by the 
Society of Exploration Geophysicists (SEG), the Candadia Exploration Geophysical
Society (KEGS), and the Australian Society of Exploration Geophysicists (ASEG) 
as the standard format for exchanging gridded gravity and magnetic data.  A 
complete description of GXF is provided in the document gxf.pdf provided by 
Geosoft, Inc. (http://www.geosoft.com).

To speed file transfers, both files have additionaly been compressed into the 
public-domain "gzip" format (as indicated by the suffix .gz).  Gzip format is 
commonly used on PC DOS, PC Windows, Macintosh, and Unix platforms.  Information 
on gzip is available at

http://www.maths.lancs.ac.uk/~smithdm1/GNU/GNUWeb/gzip.html 
   (unix and PC DOS versions)

 http://www.winzip.com/ 
   (PC windows version)

 http://www.1source.com/tools/compress.html
   (contains link to download Mac gzip)