David L. Daniels, Robert P. Kucks, Patricia L. Hill, and Stephen L. Snyder
USGS Data Series 411


(1) Gridded Data Transfer:

The grids are presented in Geosoft Oasis montaj (proprietary software) binary format, 
with two files describing each of the grids (suffixes .grd and .grd.gi). The .gi file contains
information about the projection used by the software. This grid format can be converted
to numerous other formats using the downloadable free software offered by Geosoft at 
<http://www.geosoft.com>(look for download link). Below is the projection information
for each grid as defined in the Geosoft format.

--------------------------------------------------
Projection               Lambert Conformal Conic 
Parameters:
1st standard parallel  33  0 0.000
2nd standard parallel  45  0 0.000
central meridian       -84  0 0.00
latitude of projection's origin   0  0 0.000
false easting (meters)     0.00000
false northing (meters)    0.00000
--------------------------------------------------


(2) Displaying .map files

The files with file type .map are also in the Geosoft Oasis montaj format and can be displayed only
with Geosoft's Oasis montaj free reader software. The advantage of the .map files over plotting a grid
file is that all the data used (data points, coordinate ticks, state boundaries and shorelines, scale 
bar, color bar, title, and other plotted data) will be displayed on the screen and also can be plotted.


(3) Explanation of headers for gravity point data files (.xyz and .csv):

Latitude  degrees N
Longitude degrees W
Elev_m    elevation above mean sea level in meters
Gravity   observed gravity in milligals
Faa       free air anomaly in milligals
Itc       inner terrain correction in milligals
Otc       outer terrain correction in milligals
Sba       simple Bouguer anomaly in milligals
Cba       complete Bouguer anomaly in milligals
Source    gravity survey number