Robert P. Kucks and Patricia Hill APRIL 2000
README_TXT FILE FOR GRIDDED DATA TRANSFER DIRECTORY
This directory contains sub-directories for ascii and binary
gridded files of the Wyoming merged aeromagnetic and gravity data.
Each data grid is available in 4 different data formats:
1. GXF - GeoSoft GXF format (an ascii grid transfer format)
2. ERM - ER Mapper grid format (a two-file transfer format)
3. ARC - ARC/INFO grid export format (a binary format)
4. ODDF - USGS ODDF (version 1.6.1) 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 the GXF and ARC directories 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 all gzip files. For example:
original filename: wy_magfin.e00
changed to: wy_magfin_e00
compressed file name: wy_magfin_e00.gz
After you transfer and uncompress the files, rename them to
replace the "_" with a "." again.
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Projection Lambert Conformal Conic
Parameters:
1st standard parallel 33 0 0.000
2nd standard parallel 45 0 0.000
central meridian -107 30 0.00
latitude of projection's origin 41 0 0.000
false easting (meters) 0.00000
false northing (meters) 0.00000
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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:
ą1 first point at bottom left of grid
ą2 first point at upper left of grid
ą3 first point at upper right of grid
ą4 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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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 as "RAW", indicating that the projection information
and format must be defined by the user. The ERM directory also
contains two files (project.dat and lamcon2.dat) in ER Mapper format
that describe projection parameters.
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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.
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4. USGS ODDF binary grid format (*_grd)
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.
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