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 seperated by a carraige return.

In order 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:

    NCOLS xxx
    NROWS xxx
    XLLCENTER xxx | XLLCORNER xxx
    YLLCENTER 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. The definitions of the kewords are:

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 ...