Slope is the measure of change in surface value over distance
and can be expressed in degrees or as a percentage. In a raster
format, the Digital Elevation Model (DEM) is a grid where
each cell is a value referenced to a common datum. For extraction
of elevation from remote sensing dedicated software packages
are required but most GIS packages have routines for
point or contour line interpolation. Any two points on the grid
will be sufficient to ascertain a slope. Once the slopes have been
calculated, then the maximum difference can be found and the
gradient can be determined