**EXPLANATION FILE FOR GRIDDED DATA**

The magnetic and gravity grids are available in different data formats:

1\. Geosoft (.GRD) format (a binary format)

2\. Grid Exchange Format (.GXF) (an ascii grid transfer format)

3\. USGS (.USG) (a binary format)

Each of these formats is discussed briefly below. If none of the formats
is directly readable by software on your system, then the GXF format is
most easily modified by inputting and

translation. Extensive description of this format is given below.

All of the grids use the same projection parameters as follows:

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Projection Lambert Conformal Conic

Parameters:

1st standard parallel 33 degrees

2nd standard parallel 45 degrees

central meridian -81 degrees

latitude of projection\'s origin 0 degrees

false easting (meters) 0.00000

false northing (meters) 0.00000

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1\. Geosoft grid (\*.grd)

Binary grid format used with Geosoft\'s Oasis Montaj. A free viewer that
will display and print to hard copy may be downloaded from:
http://www.geosoft.com/pinfo/free/index.html

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2\. Grid eXchange Format (\*.gxf)

GXF (Grid eXchange File) is a standard ASCII file format for exchanging
gridded data among different software systems. Software that supports
the GXF standard will be able to import

properly formatted GXF files and export grids in GXF format.

Grid Description:

A grid is a rectangular array of points at which single data values
define a two dimensional function. Grid point locations are related to a
Grid Coordinate System (GCS), which is a right

handed Cartesian system with X and Y axis defined by the bottom and left
sides of a grid array. The grid point at the bottom, left corner of the
array is the origin of the GCS. All distances

are in meters.

GCS coordinates are related to a Base Coordinate System (BCS) through a
plane translation and rotation. The origin of the GCS is located at
point (x0,y0) in the BCS, and the X and Y grid

indices are related to BCS units through the separation between points
in the GCS X and Y directions.

Labeled Data Objects and Comments

A GXF file is an ASCII file made up of a number of labeled data objects
and comments. Each labeled data object has a label line followed by one
or more data lines. A label line is identified

by a \'#\' character in the first column followed immediately by an
upper-case label. The data associated with that label are found on one
or more lines that follow the label.

Lines

All lines in a GXF file must be less than or equal to 80 characters in
length. Any lines that are not part of a labeled data object are ignored
and can be used to place comments within a GXF file. Programs that read
GXF files will skip such comment lines while they search for the next
GXF data object.

GXF Object Definitions

#TITLE

A one line descriptive title of the grid. Some grid formats include
textual descriptions of the grid, and this information can be placed in
a #TITLE object.

Default: blank title

#POINTS

The number of points in each grid row (horizontal or vertical as defined
by the #SENSE object).

Default: no default - this object is required.

#ROWS

The number of rows in the grid. A grid row (or vector) is a collection
of consecutive grid points that represent the grid values along a
horizontal or vertical line in the grid. The complete grid is then
defined by a consecutive sequence of grid rows.

Default: no default - this object is required.

#PTSEPARATION

The separation between points in the grid. This should be in Base
Coordinate System units (ground units for geographically based grids).

Default: 1.0

#RWSEPARATION

The separation between rows in the grid. These should be in Base
Coordinate System units (ground units for geographically based grids).

Default: 1.0

#XORIGIN

The X location of the bottom left corner of the grid in the Base
Coordinate System.

Default: 0.0

#YORIGIN

The Y location of the bottom left corner of the grid in the Base
Coordinate System.

Default: 0.0

#ROTATION

The rotation angle of the grid. This is the counter-clockwise angle of
the bottom edge of the grid with respect to the Base Coordinate System X
axis. Rotation only has meaning for Base

Coordinate Systems that use the same units on the X and Y axis.

Default: 0.0

#SENSE

The first point of the first row of the stored grid can be at any corner
of the grid rectangle, and the grid rows can be run vertically or
horizontally. The SENSE object defines this storage sense as follows:

±1 first point at bottom left of grid

±2 first point at upper left of grid

±3 first point at upper right of grid

±4 first point at bottom right of grid

A positive SENSE stores rows in a right-handed sense; a negative SENSE
stores rows in a left-handed sense. This means that if you were standing
at the first grid point and looking into the grid,

the first grid row would extend to your right for a right handed grid
(positive sense), or to your left for a left handed sense (left-handed
grid): (All grids on this CD have SENSE=+1.)

Default: 1 (first point at bottom left, rows left to right)

#TRANSFORM

This keyword is followed by two numbers on the same line: SCALE and
OFFSET, which are used to transform the grid data to desired units: Z =
G \* SCALE + OFFSET

where

Z grid value in the desired unit

G are grid values as specified in the #GRID object

Default: SCALE = 1.0, OFFSET = 0.0

#DUMMY

The grid must be rectangular (every row must have the same number of
points). The dummy value defined by this object is used to define blank
areas of the grid. Any grids that include blank areas must define a
dummy value.

Default: no dummy value.

#GRID

The grid data is listed point by point and row by row. The #GRID object
and data is always the last object in a GXF file. The first data point
is at the location indicated by #SENSE, and is followed by successive
points in that row of points (either horizontal or vertical), then the
points in the next row, and so on. The points in a row can follow on to
the next data line, although each new row must start on a new data line.
A GXF reading program can expect #ROWS of #POINTS for a total of #ROWS
times #POINTS data values.

Default: none, must be included as the last object in a GXF file.

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3\. USGS PC binary grid format (\*.usg)

This is a binary format compatible with the USGS PC Potential Field

software. The free software package may be downloaded from:

ftp://greenwood.cr.usgs.gov/pub/open-file-reports/ofr-97-0725/pfofr.htm.

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