U.S. Geological Survey

Get Data: File Type Explanation

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Rick Saltus
13 August 1998

The following directory contains ASCII and binary gridded files for
the Alaska aeroradiometric data.  Each data grid is available
in four different data formats:

1. GeoSoft GXF format (an ASCII grid transfer format)(nure_*.gxf)
2. ER Mapper grid format (a two-part transfer format)(nure_* & nure_*.ers)
3. ARC/INFO grid export format (a binary format)(nure_*.e00)
4. USGS ODDF grid format (a binary format)(nure_*.gd)

Each of these formats is discussed briefly below.
If none of these formats are directly readable by software on your
system, then your best bet is probably to write a program to input
and translate the GXF format.  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, I have replaced the 
original "." in the filenames with an "_" (underscore) character.  
For example:

original filename: nure_k.e00
changed to: nure_k_e00
compressed file name: nure_k_e00.gz

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

----------------------------------------------------------------------------
BASIC GRID STATISTICS (All Grids)

Cell Size =            2000.000 meters  Data Type: Floating Point
Number of Rows    =        855
Number of Columns =        1110

           BOUNDARY 

Xmin =              -904000.000 meters  
Xmax =              1316000.000 meters  
Ymin =               120000.000 meters  
Ymax =              1830000.000 meters  

                          COORDINATE SYSTEM DESCRIPTION

Projection               ALBERS
Units                    METERS             Spheroid             CLARKE1866
Parameters:
1st standard parallel                                   55  0  0.000
2nd standard parallel                                   65  0  0.000
central meridian                                       -151  0  0.00
latitude of projection's origin                         55  0  0.000
false easting (meters)                                       0.00000
false northing (meters)                                      0.00000
----------------------------------------------------------------------------

1. Grid eXchange Format (nure_*.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.  (For these Alaska grids, the base coordinate system
is the Albers conical equal-area projection with standard parallels of 55 and
65 degrees, a base latitude of 55 degrees north, a central meridian of 151
degrees west, a semi-major axis of ellipsoid of 6378206.400 meters, an
eccentricity of 0.0067686579973.)  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 (nure_k & nure_k.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 "ALINTAK1".  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.

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

3. ARC/INFO Grid Export format (nure_k.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 (nure_k.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.  

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