xllcorneryllcorner

xllcorner and yllcorner are header fields in the Arc/Info ASCII Grid format used by GIS software to georeference raster data. The fields specify the coordinates of the lower-left corner of the grid in the dataset’s coordinate reference system. Together with the number of columns (ncols), the number of rows (nrows), the cell size, and the NODATA value, these coordinates define the grid’s spatial footprint.

The values are floating-point coordinates that indicate where the grid begins in the map projection or geographic coordinate system. For example, a header might include xllcorner 480000.0 and yllcorner 300000.0, placing the lower-left corner of the grid at those coordinates. The overall extents of the grid can be computed as:

xmin = xllcorner

xmax = xllcorner + ncols * cellsize

ymin = yllcorner

ymax = yllcorner + nrows * cellsize

There are variations in the data format: some grids use xllcenter and yllcenter to specify the coordinates of the lower-left corner of the center of the grid cell rather than the outer corner. Understanding the distinction is important for accurate spatial alignment, especially when integrating with other raster or vector data.

Significance: xllcorner and yllcorner are essential for converting a raster’s row-column indices to real-world coordinates, enabling GIS analyses, map rendering, and data integration. They are widely documented in ASCII grid specifications and are commonly encountered in land cover, elevation, and climate datasets.