7.5-Minute Quadrangle DRGs

USGS 7.5-Minute Quadrangle Digital Elevation Maps (DEM's)

Description: The 7.5-minute (30m) DEM is derived either by digitizing USGS 1:24,000-scale quadrangle maps (which gives less accurate results) or by scanning aerial photographs (now exclusively using the National Aerial Photography Program's quadrangle-centered photographs). The vertical scale of the 7.5-minute DEM's may be in feet (in areas of lower relief) or in meters (in areas of higher relief). Many of the 7.5-minute DEM's (notably those captured by digitizing existing map sheets) are regarded as being of relatively poor quality, particularly in places of high relief. Also, they are often not well edgematched as a result of being produced on the basis of individual map sheets. There are significant gaps between quadrangles. Data interpolation is the only viable means of filling these gaps. The 7.5-minute DEM's are referenced to the UTM cartesian coordinate system, whereas all other USGS DEM's are referenced to geographical (latitude/longitude) coordinates.

USGS DEM Accuracy: The USGS classifies DEM accuracy into three distinct levels:

Level 1 (now photogrammetric source only) RMSE 15 m, absolute maximum error 50 m. - Most of the 7.5-minute DEM's (and all new ones) have a 7.5 m RMSE. The older digitized 15-m-acccuracy DEM's will all eventually be replaced by those of greater accuracy.
Level 2 (edited photo source, cartographic source) - RMSE less than 0.5 contour interval, absolute maximum error less than 1 contour interval.
Level 3 (cartographic source) - RMSE less than 0.333 contour interval, absolute maximum error less than 0.666 contour interval. For all USGS DEM's, the accuracy classification level is recorded within the DEM data structure.

USGS 7.5-minute topographic digital elevation models in USGS DEM format, UTM zones 16 and 17, are available from the Kentucky Geological Survey on CD-ROM. Three CD's cover the entire state. Individual CD's are $30. The DEM's are also available for free download at Kentucky Office of GIS, under DATA.

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