Raster Measurements

Measurements on raster data layers are simpler because of the regularity of the cells. Location of an individual cell derives from the raster’s anchor point, the cell resolution, and the position of the cell in the raster. Again, there are two conventions: the cell’s location can be its lower-left corner, or the cell’s midpoint. These conventions are set by the software in use, and in cases of data of low resolution it becomes more important to be aware of them.

The area size of a selected part of the raster (a group of cells) is calculated as the number of cells multiplied by the cell-area size. The area size of a cell is constant and is determined by the cell resolution. Horizontal and vertical resolution may differ, but typically they do not. Together with the location of what is called an anchor point , this is the only geometric information stored with the raster data, so all other measurements by the GIS are computed. The anchor point is fixed by convention to be the lower-left (or sometimes upper-left) location of the raster. Distance is calculated by the standard distance function.

Standard Distance Function

The distance between two raster cells is the standard distance function applied to the locations of their respective midpoints; obviously the cell resolution has to be taken into account. Where a raster is used to represent line features as strings of cells through the raster, the length of a line feature is computed as the the sum of distances between consecutive cells.