Stephen L. Snyder

USGS Open File Report 2005-1052

**GRID_FACTS.TXT FILE FOR GRIDDED DATA TRANSFER DIRECTORY**

This directory contains sub-directories for ascii and binary

gridded files of the Virginia aeromagnetic and gravity data.

The gravity point files are in the gravity directory.

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)

4\. ESRI Spatial Analyst (.flt, .hdr) 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.

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: -79 degrees

latitude of projection\'s origin: 0 degrees

false easting: 0.0 meters

false northing: 0.0 meters

Datum: NAD27

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1\. Geosoft/Oasis Format (\*.grd & \*.gi)

This binary grid format is the default standard for the Geosoft/

Oasis commercial software package. The .gi file contains the

projection information used by the software to internally define

it. A free viewer that will display and print to hard copy may

be downloaded from:

<http://www.geosoft.com/pinfo/oasismontaj/free/index.asp>

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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.

See:

<http://www.geosoft.com/resources/technotes/pdfs/gxfr3d9_1.pdf> or

<http://www.getech.com/standards.htm>

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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:

<http://pubs.usgs.gov/of/1997/ofr-97-0725/pfofr.htm>.

The standard file suffix is .grd for this USGS software. The suffix .usg

is used here to distinguish this grid type from the Geosoft grid format

which also uses the suffix .grd.

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4\. ESRI Spatial Analyst (.flt, .hdr) binary format

This is a binary format that can be imported into ESRI\'s Arc View
through

the Spatial Analyst software. Basic grid parameters are contained in the

ascii \*.hdr file.

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