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. ---------------------------------------------------------- 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 ----------------------------------------------------------------- 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 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. ----------------------------------------------------------------- 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 (*_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. -----------------------------------------------------------------