Viki Bankey, Tien Grauch, and Bob Kucks      Nov 1998

README.TXT FILE FOR GRIDDED AND POINT DATA TRANSFER DIRECTORY

This directory contains ascii and binary gridded files for the
Utah merged aeromagnetic and gravity data. Each data grid is
available in  different data formats:

1. GeoSoft GXF format (an ascii grid transfer format)
2. ER Mapper grid format (a two-file transfer format)
3. ARC/INFO grid export format (a binary format)
4. USGS ODDF grid format (a binary format)

Each of these formats is discussed briefly below. If none of
these 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.

The data grids in this directory have been compressed using the
public- domain "gzip" compression utility.  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)

To avoid file name problems on older PC systems, the  original
"." in the filenames has been replaced with an "_" (underscore)
character.  For example:

original filename: utahcomp.e00
changed to: utahcomp_e00
compressed file name: utahcomp_e00.gz

After you transfer and uncompress the files, rename them to
replace the "_" with a "." again.

----------------------------------------------------------
Projection               Lambert Conformal Conic 
Parameters:
1st standard parallel  33  0 0.000
2nd standard parallel  45  0 0.000
central meridian       -112  0 0.00
latitude of projection's origin   0  0 0.000
false easting (meters)     0.00000
false northing (meters)    0.00000
-----------------------------------------------------------------

1. 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.

-----------------------------------------------------------------

2. ER Mapper grid format (*.ers)

The ER Mapper grid format consists of two files: a binary data
file  (no file suffix) and a ascii header file (*.ers).  These
files can be read directly by the ER Mapper software or by other
packages such as ERDAS IMAGINE.  Note that the header files refer
to the projection called "L2UTMAG".  This is not a standard ER
Mapper projection.  To register the grid properly within ER
Mapper, you must create a  projection entry that corresponds to
the parameters listed above for the projection of these data. 
The files "project.dat" and "lamcon2.dat" include the correct
projection information and format.

-----------------------------------------------------------------

3.  ARC/INFO Grid Export format (*.e00)

This format is readable by ARC/INFO, ERDAS IMAGINE, and other
packages.  When imported into ARC/INFO, this file will unpack
into a directory containing all the components of the registered
grid coverage.

-----------------------------------------------------------------

4. USGS ODDF binary grid format (*.gd)

This is a binary format with an included ascii header.  This
format is used by the USGS Geophysics Group within the Minerals
Program of the Geologic Division.   The ascii header gives all
information necessary to convert this file to another format.

-----------------------------------------------------------------

5.  ASCII Gravity Data File (utah_boug_asc.gz)

Description of the ASCII gravity station listing, named
utah_boug_asc.gz, a compressed file containing 41,960 stations
within or adjacent to Utah. 
The fields are listed in this order:

Station identification; 8 digit numbers are from the Defense
Mapping Agency and the first four digits are their reference to
individual surveys; other stations are USGS or other sources.

Longitude, negative west, in degrees, decimal degrees

Latitude, positive north, in degrees, decimal degrees

Free air gravity anomaly, in milligals (mgals)

Bouguer gravity anomaly, in milligals, reduction density 2.67
g/cc

Elevation, in feet

Inner terrain correction zone, from 0 to .895 km radial distance,
mgals

Outer terrain correction zone, from .895 to 167 km radial
distance, mgals (note: total terrain corection obtained by adding
inner and outer zones)

Observed gravity minus 980,000. mgals, in mgals