30-meter resolution Digital
Elevation Models for Delaware
and Chester County, PA
A digital elevation model (DEM) contains surface elevation values for regular grid points interpolated from rasterized hypsography (elevation contour line) data. SpatLab has developed 30-meter resolution (1:24,000 scale) DEM's for 52 7.5-minute quads covering the state of Delaware and three additional quads in Chester Co., PA.
DEM's for 51 quads were interpolated from rasterized 1992-93 USGS hypsography data combined with zero-elevation contours for coastal quads extracted from 1989 Landsat TM Band 7 satellite imagery. DEM's for four quads (West Grove, Coatesville, Morgantown and Bay View) were created by scanning hardcopy chromopaque separates obtained from USGS, vectorizing and labeling the contour lines to create digital hypsography files, re-rasterizing and interpolating these to obtain DEM's. Additional DEM's for Chester Co., PA, obtained from USGS, are also available here.
Like the source hypsography, the original 7.5-minute DEM's are in UTM-18N projection, North American Datum 1983 based on GRS80 spheroid. Z-values in all DEM's are integer feet. DEM's created by us are exactly 30x30M resolution with good edge consistency due to inclusion of hypsography from neighboring quads in the DEM interpolation process; edges of these DEM's extend slightly beyond actual quad boundaries. The DEM's created by USGS extend to quad boundaries more precisely, but have slightly inconsistent cell resolution and some discernible discontinuities at quad boundaries.
We are distributing these by quad from a clickable map in UTM-18N, NAD83 projection only, and composited by county in both UTM and DE State Plane (NAD83, meters) projections.
"Filled" versions of the DEM's composited by county are also available for hydrologic modeling. These DEM's have been processed to fill in all "pits" and other local depressions, so that every cell in the map flows consistently to an outlet at the map edge. These filled DEM's can be analyzed by flood-fill algorithms to delineate watersheds, model runoff volumes, etc.
In 1999 we experimented with developing 10-meter resolution DEM's based on the 1992-93 USGS hypsography and new USGS hydrography files. Check out West Grove (~900MB gzipped ASCII, see notes for header editing). Given the scale and resolution of the source hypsography and the low relief of Delaware's coastal plain, we believe that most of the purported accuracy gain in 10-meter resolution DEM's is specious. We may complete a 10-meter resolution series for the piedmont region of Northern New Castle County, but will not be issuing a statewide series.
If you have questions about these data, please e-mail John Mackenzie.
DEM's copyright J. Mackenzie,
These data are distributed free of charge and may be freely redistributed with appropriate citation.
We disclaim any and all liability resulting from any errors in these data, or from any use or misuse of these data.