Frequently Accessed Data Layers
A collection of commonly used GIS datasets for Tennessee. Most data are
maintained in Tennessee State Plane, NAD83, Units Feet, to enable
overlay and integration functions, and are organized by theme.
These data sets are derived from the U.S. Census
Bureau's 2000 Redistricting version of
the TIGER line files. The population data associated or joined with these
files are derived from the Census 2000
Redistricting Data (P.L. 94-171) Summary File. The data
includes total and voting age population by
racial category at the county, city, and block level geography. Other
planimetric layers are also included.
employ the Stateplane coordinate system, NAD 83, Feet. For more
details, please view the following
Digital Orthophoto Quadrangle (DOQQ)
Orthophotos combine the image
characteristics of a photograph with the geometric qualities of a map. The
primary digital orthophotoquad (DOQ) is a 1-meter ground resolution,
quarter-quadrangle (3.75-minutes of latitude by 3.75-minutes of longitude)
image cast on the Universal Transverse Mercator Projection (UTM) on the
North American Datum of 1983 (NAD83).
DOQ's serve a variety of purposes, from interim maps to field references for
earth science investigations and analysis. The DOQ is useful as a layer of a
geographic information system and as a tool for revision of digital line
graphs and topographic maps.
Digital Raster Graphic (DRG)
The Digital Raster Graphic (DRG) is a raster image of a scanned USGS
topographic map excluding the collar information. The source maps for DRGs
were georeferenced to the UTM grid, but DRG images for Tennessee are
available in Tennessee State Plane, NAD83, feet. A DRG is useful as a source
or background layer in a GIS, as a means to perform quality assurance on
other digital products, and as a source for the collection and revision of
DLG data. DRG's can also be merged with other digital data, e.g. DEM's or
DOQ's, to produce a hybrid digital file.
Digital Elevation Model (DEM)
Digital elevation model (DEM) data
are arrays of regularly spaced elevation values referenced horizontally
either to a Universal Transverse Mercator (UTM) projection or to a
geographic coordinate system. The grid cells are spaced at regular intervals
along south to north profiles that are ordered from west to east. The U.S.
Geological Survey (USGS) produces five primary types of elevation data:
7.5-minute DEM, 30-minute DEM, 1-degree DEM, 7.5-minute Alaska DEM, and
15-minute Alaska DEM.