A Digital Elevation Model (DEM) is created by projecting vector hypsography (elevation contour) lines onto a regular spatial grid and then interpolating elevations for all cells in the grid.
Above: (1) Vector Hypsography, Newark East (DE) 7.5-minute quad: vector data extracted from 1993 USGS Digital Line Graph files;
(2) Raster Hypsography, 30x30 meter cell resolution, Newark East quad and portion of Newark West quad.
Above: Interpolated DEM, City of Newark, DE. Patched portions of Newark East and West quads. Vector hydrography (water features) in blue, interstate highway (I-95) in red, other roads in black and rail lines in white. Iron Hill in southwest corner, White Clay Creek in northwest. (Vector features extracted from 1992 Census TIGER data.)
Above: (1) Slope in degrees, calculated from DEM using 3x3 cell neighborhood, Newark East quad.
(2) Aspect (terrain orientation), calculated from DEM using 3x3 cell neighborhood, illuminated from the Northwest, Newark East quad.
(3) Aspect-enhanced DEM created using DEM for hue and Aspect for color intensity, Newark East quad.
DEM's are used extensively for digital analysis of watersheds.
GRASS info (CERL)
USGS home page
USGS on-line data
SPATLAB home page