County-wide DEM's were interpolated from tiled raster hypsography files; and should have no discernible patch lines.  Elevation values are in integer feet.  All files are ASCII format.

UNIX gzipped versions are GRASS ASCII format, UTM-18N (NAD83) projection.  Use gunzip to uncompress.  Import to GRASS with the v.in.ascii module:
 

New Castle Co. DEM (UTM)--GRASS	New Castle Co. FILLED DEM (UTM)--GRASS
Kent Co. DEM (UTM)--GRASS	Kent Co. FILLED DEM (UTM)--GRASS
Sussex Co. DEM (UTM)--GRASS	Susex Co. FILLED DEM (UTM)--GRASS
Christina Watershed DEM (UTM)--GRASS

Windows WinZipped versions are ArcINFO/ArcView grid files, both UTM-18N and DE State Plane (NAD83, meters) projections. Use WinZip to uncompress.  Import to ArcINFO with the ASCIIGRID module.  Import to ArcView (with Spatial Analyst extension loaded and a View active) with the File--Import Data Source utility (file type is ASCII Raster):
 

New Castle DEM (UTM)	New Castle FILLED DEM (UTM)
New Castle DEM (DE State Plane NAD83)	New Castle FILLED DEM (DE State Plane NAD83)
Kent DEM (UTM)	Kent FILLED DEM (UTM)
Kent DEM (DE State Plane NAD83)	Kent FILLED DEM (DE State Plane NAD83)
Sussex DEM (UTM)	Sussex FILLED DEM (UTM)
Sussex DEM (DE State Plane NAD83)	Sussex FILLED DEM (DE State Plane NAD83)
Christina Watershed DEM (UTM)	Christina Watershed FILLED DEM (UTM)