Redbex supports importing DEM data from files in ESRI ASCII raster format also known as ARC/INFO ASCII GRID. This file format is widely used in the GIS world as an exchange, or export format, for raster data.

The file defines the geographic space as an array of equally sized square grid points arranged in rows and columns. Each grid point stores a numeric value that represents a geographic attribute (in this case the elevation) for that unit of space.

The file begins 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. The values use the point as decimal separator.

Header format

The header of the file contains a list of keyword value pairs separated by carriage return. Keyword and value are separated by one or more space characters. Table one lists the recognized keywords.  All other keywords will be ignored. Keywords are case insensitive. The value in the keyword value pair is always a numeric value.

The parameters in the header part of the file must match correctly with the structure of the data values.

Table 1: Keywords in the header of a ESRI ASCII grid file.

Data format

The data component of the file follows the header information. The first data line is the first line in the file that does not start with an alphabetic character (A-Z).

Cell values are delimited by spaces. No carriage returns are necessary at the end of each row in the raster. The number of columns defined 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.

Following rules apply when parsing numbers:

•The numeric string can have a leading sign, + for positive and - for negative values.

•The numeric string can include a decimal separator, the decimal separator can be point or comma but must be the same throughout the whole file.

•The numeric string can be in exponential notation. The numeric string may contain an exponent that begins with the E or e character and that is followed by an optional positive or negative sign and an integer. In other words, it successfully parses strings in the form nnnExx , nnnE+xx , nnnE-xx

Listing 1: Example of an ESRI ASCII grid file (truncated)