ST_Roughness — Returns a raster with the calculated "roughness" of a DEM.
raster ST_Roughness(raster rast, integer nband, raster customextent, text pixeltype="32BF" , boolean interpolate_nodata=FALSE );
Calculates the "roughness" of a DEM, by subtracting the maximum from the minimum for a given area.
Availability: 2.1.0
-- needs examples