xllcorner

xllcorner is a header keyword used in the Esri Arc/Info ASCII Raster format to describe the position of a raster’s grid in geographic space. It specifies the x coordinate of the lower-left corner of the raster’s lower-left cell, together with yllcorner for the vertical coordinate. Together, xllcorner and yllcorner define the origin of the grid in the chosen coordinate reference system.

In a typical ASCII grid header, the fields ncols and nrows define the grid size in cells, followed by xllcorner (or xllcenter) and yllcorner (or yllcenter), then cellsize and NODATA_value. The xllcorner and yllcorner designate the origin at the lower-left; if the file uses xllcenter and yllcenter, the coordinates refer to the center of the lower-left cell. This distinction affects how the raster aligns with real-world coordinates and how new grids are generated or projected.

Example header:

ncols 120

nrows 80

xllcorner 378923.0

yllcorner 4072345.0

cellsize 30.0

NODATA_value -9999

The coordinates in xllcorner are expressed in the raster’s coordinate system, which may be geographic or projected. ASCII grids are simple and portable, making them easy to share, but they are text-based and less space-efficient than binary formats. Most GIS software can read or convert Arc/Info ASCII grids to other formats such as GeoTIFF, preserving the stated origin regardless of the chosen xllcorner or xllcenter convention.