ARC ASCIIGRID refers to a specifc interchange format developed for ARC/INFO rasters in ASCII format. The format consists of a header that specifies the geographic domain and resolution, followed by the actual grid cell values. Usually the file extension is .asc, but recent versions of ESRI software also recognize the extension .grd. It looks like this:
Records 1 - 6 Geographic header
Coordinates may be in decimal or integer format. DD:MM:SS format for geodetic coordinates is not supported.
ncols refers to the number of columns in the grid and xxxxx is the numerical value
nrows refers to the number of rows in the grid and xxxxx is the numerical value
xllcorner refers to the western edge of the grid and xxxxx is the numerical value
yllcorner refers to the southern edge of the grid and xxxxx is the numerical value
cellsize refers to the resolution of the grid and xxxxx is the numerical value
nodata_value refers to the value that represents missing data and xxxxx is the numerical value. This is
optional and your parser should not assume it will be present. Note: that if you need a good value, the ESRI default is -9999.
Record 7 -> end of file Data values
These are the value of individual cell typically representing elevation of a particular area.
xxx xxx xxx
val(nox,noy) (f) = individual grid values, column varying fastest in integer format. Grid values are stored as integers but can be read as floating point values.
xllcorner and yllcorner are given as the EDGES of the grid, NOT the centers of the edge cells. ARC/INFO supports other header strings that allow the centers of the edge cells to be given using xllcenter and yllcenter instead. The origin of the grid is the upper left and terminus at the lower right.
ARC format grids are single-band files.