ESRI ASCII raster format
The ESRI ASCII raster format can be used to transfer information to or from other cell-based or raster systems. When an existing raster is output to an ESRI ASCII-format raster, the file will begin with header information that defines the properties of the raster such as the cell size, the number of rows and columns, and the coordinates of the origin of the raster. The header information is followed by cell value information specified in space-delimited row-major order, with each row separated by a carriage return.
To convert an ASCII file to a raster, the data must be in this same format. The parameters in the header part of the file must match correctly with the structure of the data values.
The basic structure of the ESRI ASCII raster has the header information at the beginning of the file followed by the cell value data:
The basic structure of the ESRI ASCII raster has the header information at the beginning of the file followed by the cell value data. The spatial location of the raster is specified by the location of the lower left cell, and either by:
- The center of the lower left cell
NCOLS xxx NROWS xxx XLLCENTER xxx YLLCENTER xxx CELLSIZE xxx NODATA_VALUE xxx row 1 row 2 ... row n
- The lower left corner of the lower left cell
NCOLS xxx NROWS xxx XLLCORNER xxx YLLCORNER xxx CELLSIZE xxx NODATA_VALUE xxx row 1 row 2 ... row n
Row 1 of the data is at the top of the raster, row 2 is just under row 1, and so on.
Header format
The syntax of the header information is a keyword paired with the value of that keyword. These are the definitions of the keywords:
|
Parameter |
Description |
Requirements |
|---|---|---|
|
NCOLS |
Number of cell columns |
Integer greater than 0. |
|
NROWS |
Number of cell rows |
Integer greater than 0. |
|
XLLCENTER or XLLCORNER |
X-coordinate of the origin (by center or lower left corner of the cell) |
Match with y-coordinate type. |
|
YLLCENTER or YLLCORNER |
Y-coordinate of the origin (by center or lower left corner of the cell) |
Match with x-coordinate type. |
|
CELLSIZE |
Cell size |
Greater than 0. |
|
NODATA_VALUE |
The input values to be NoData in the output raster |
Optional. Default is -9999. |
Data format
The data component of the ESRI ASCII raster follows the header information.
- Cell values should be delimited by spaces.
- No carriage returns are necessary at the end of each row in the raster. The number of columns in the header determines when a new row begins.
- Row 1 of the data is at the top of the raster, row 2 is just under row 1, and so on.
Example ASCII raster:
ncols 480 nrows 450 xllcorner 378923 yllcorner 4072345 cellsize 30 nodata_value -32768 43 2 45 7 3 56 2 5 23 65 34 6 32 54 57 34 2 2 54 6 35 45 65 34 2 6 78 4 2 6 89 3 2 7 45 23 5 8 4 1 62 ...