A Digital Elevation Model (DEM) is a representation of the terrain using point elevation information. A gridded DEM is one in which the elevation points are spaced at regular intervals so as to create a grid or lattice. These grids can be directly observed, but generally they are computed from other elevation information such as contours or irregularly spaced spot heights.
GEODATA 9 Second DEM and D8 Flow Direction Grid 2008
The D8-9S Flow Direction Grid is a corresponding grid describing the principal directions of surface drainage across the whole of Australia.