Introduction | Geomorphology | Raster data files | Vector data files | Images

RASTER FILE INFORMATION

GMT/NetCDF

The "grd" files are in the hardware-independent, NetCDF format used by the Generic Mapping Tools (GMT) graphics software. GMT is a freeware package of C language graphics tools designed for use in the UNIX environment. GMT tools make graytone and color PostScript output for scientific visualization and publication-quality maps and graphs. GMT is distributed free of charge over the Internet by anonymous ftp from kiawe.soest.hawaii.edu (128.171.151.16) in the /pub/gmt directory. NetCDF is a package of I/O routines that allow hardware-independent data storage using XDR plus a description and retrieval layer; it is available by ftp from unidata.ucar.edu (128.117.140.3).

If you are using GMT the binary "grd" files will be readable directly, and you will not need to write your own code to read them.

The data files are pixel registered (in the sense used in GMT). In this registration, the nodes are centered in the grid cells which are defined by grid lines. The grid for these data are on interger multiples of 3 (or9 or 30) arc seconds of latitude and longitude. Further, the value of the node point represents the average value within the entire cell.

The 3 arc-second grd file (mich_03.grd) is a grid with 4300 values in the X direction or longitude, and 5700 values in the Y direction or latitude. The grid spans from 88o 05' west to 84o 30' east and from 41o 30' south to 46o 15' north.

The 9 arc-second grd file (mich_09.grd) is a grid with 1433 values in the X direction or longitude, and 1900 values in the Y direction or latitude. The grid spans from 88o 05' west to 84o 30' east and from 41o 30' south to 46o 15' north.

The 30 arc-second grd file (mich_30.grd) is a grid with 430 values in the X direction or longitude, and 570 values in the Y direction or latitude. The grid spans from 88 05' west to 84 30' east and from 41 30' south to 46 15' north.

Binary grid

The binary grid files are two-byte signed integer values in the Sun byte-order (twos-complement, big- endian, or most significant byte first). Vax and Intel chips use little-endian, so IBM PCs and VAXes will have to swap the bytes to use these files; Macs, Suns, SGIs, HPs, newerDECs, etc. should be able to use them as-is.

The data files are pixel registered (in the sense used in GMT). In this registration, the nodes are centered in the grid cells which are defined by grid lines. The grid for these data are on interger multiples of 3 (or9 or 30) arc seconds of latitude and longitude. Further, the value of the node point represents the average value within the entire cell.

This description should enable you to write your own code to read the 'bin' files. The file describes an array of x rows and y columns. The sequence of the array elements is in "scan-line order", which is written by rows, from the top row to the bottom row, and within each row is written from left to right. The latitude and longitude, in decimal degrees, of the center of a pixel can be described this way in C using the 3 arc-second file mich_03.bin as an example:

	short int       data[24510000];
	FILE            *filepointer; 
	int             ij, jrow, icolumn; 
	double          xlongitude[4300], ylatitude[5700]; 

	for (jrow = 0; jrow < 5700; jrow++) 
		ylatitude[j] = 46.25 - 0.000416667 * (jrow + 0.5); 
 
	for (icolumn = 0; icolumn < 4300; icolumn++) 
		xlongitude[icolumn] = 0.000833333 * (icolumn + 0.5); 
 
	if ( (filepointer = fopen("mich_03.bin", "r")) ++ NULL) { 
		/* Error condition  */ 
	} 
 
	for (ij = 0, jrow = 0; jrow < 5700; jrow++) { 
	  for (icolumn = 0; icolumn < 4300; icolumn++, ij++) { 
	    if ( (fread((char *)&data[ij], 2, 1, filepointer)) != 1) { 
		/* Error condition  */ 
	    } 
	    /* Here xlongitude[icolumn], ylatitude[jrow], and data[ij] 
		correspond to one another.  */ 
	  } 
	} 
 
	fclose(filepointer); 

The 3 arc-second binary file (mich_03.bin) is a grid with 4300 values in the X direction or longitude, and 5700 values in the Y direction or latitude. The grid spans from 88o 05' west to 84o 30' east and from 41o 30' south to 46o 15' north.

The 9 arc-second binary file (mich_09.bin) is a grid with 1433 values in the X direction or longitude, and 1900 values in the Y direction or latitude. The grid spans from 88 05' west to 84 30' east and from 41 30' south to 46 15' north.

The 30 arc-second binary file (mich_30.bin) is a grid with 430 values in the X direction or longitude, and 570 values in the Y direction or latitude. The grid spans from 88o 05' west to 84o 30' east and from 41o 30' south to 46o 15' north.

ASCII grid

The data files are pixel registered (in the sense used in GMT). In this registration, the nodes are centered in the grid cells which are defined by grid lines. The grid for these data are on interger multiples of 3 (or 9 or 30) arc seconds of latitude and longitude. Further, the value of the node point represents the average value within the entire cell. The first value corresponds to the northwest corner of the area. The sequence of the array elements is in "scan-line order", which is written by rows, from the top row to the bottom row, and within each row is written from left to right. A data value of -9999 indicates no data were available for that grid cell.

The 3 arc-second ASCII grid file (mich_03.asc) is a raster grid with 4300 values in the X direction or longitude, and 5700 values in the Y direction or latitude. The grid spans from 88o 05' west to 84o 30' east and from 41o 30' south to 46o 15' north.

The 9 arc-second ASCII grid file (mich_09.asc) is a raster grid with 1433 values in the X direction or longitude, and 1900 values in the Y direction or latitude. The grid spans from 88o 05' west to 84o 30' east and from 41o 30' south to 46o 15' north.

The 30 arc-second ASCII grid file (mich_30.asc) is a raster grid with 430 values in the X direction or longitude, and 570 values in the Y direction or latitude. The grid spans from 88o 05' west to 84o 30' east and from 41o 30' south to 46o 15' north.

ASCII XYZ grid

The data files are pixel registered (in the sense used in GMT). In this registration, the nodes are centered in the grid cells which are defined by grid lines. The grid for these data are on interger multiples of 3 (or9 or 30) arc seconds of latitude and longitude. Further, the value of the node point represents the average value within the entire cell. The first value corresponds to the northwest corner of the area. The sequence of ASCII xyz (longitude, latitude, thickness) triplets is in "scan-line order", which is written by rows, from the top row to the bottom row, and within each row is written from left to right. A data value of -9999 indicates no data were available for that grid cell.

The 9 arc-second ASCII xyz file (mich_09.xyz) is an array of xyz triplets with 1433 values in the X direction or longitude, and 1900 values in the Y direction or latitude. The grid spans from 88o 05' west to 84o 30' east and from 41o 30' south to 46o 15' north.

The 30 arc-second ASCII xyz file (mich_30.xyz) is an array of xyz triplets with 430 values in the X direction or longitude, and 570 values in the Y direction or latitude. The grid spans from 88o 05' west to 84o 30' east and from 41o 30' south to 46o 15' north.

Introduction | Geomorphology | Raster data files | Vector data files | Images