This coverage contains Hydrologic Unit Areas and their associated coding for Tennessee watersheds. The boundaries of these areas were hand delineated at a scale of 1:24,000 and then digitized on screen over DRG's obtained from the Tennessee Valley Authority.
publication date
Internal feature number.(Source:
ESRI)
Feature geometry.(Source:
ESRI)
Feature geometry.(Source:
ESRI)
These watershed and sub-watershed hydrologic unit boundaries provide a
uniquely identified and uniform method of subdividing large drainage
areas. These smaller sized hydrologic units are useful in many
programs supported by the Natural Resources Conservation Service
and others public and private entities involved in natural resource work.
This data set is intended as a tool for water-resource management and
planning activities, particularly for site-specific and localized
studies which require the amount of detail provided by this data
This data set was developed between 1995 to 1999 following National
NRCS procedures as outlined in National Instruction Number 170-304
and its related updates.
The HUA boundaries were first delineated by hand on USGS 7.5', 1:24K
topographic maps of various dates. These lines were checked by
local NRCS field personal, local groups, units of government,
or other county level entities who had an interest in the data.
Next the maps were submitted to the NRCS GIS office in Murfreesboro
Tennessee for further review and editing. The line work was then
digitized on screen over DRG's produced by the Tennessee Valley
Authority. The base map dates of the DRG's were the latest version
available as of 1994 when the DRG production was completed by TVA.
The HUA boundaries were digitized in Grass 4.13, in a NAD 83 UTM
Zone 16 or 17 mapset. HU Areas in UTM Zone 15 (extreme SW Tenn)
were digitized as if they were in UTM Zone 16. On screen digitizing
scales were normally around 1:7500, but ranged from 1:5000 to 1:15000.
This exceeded the stated accuracy of the DRGs but was done to produce
a more visually appealing line and does not imply that the data contained
is of that accuracy. This HUA line work is certified to be accurate at
1:24000. Campbell and Anderson County HUA lines were digitized in UTM
zone16 mapset and Knox and Blount County HUA lines were digitized in
a UTM zone17 mapset.
After on screen digitizing was finished all line work was plotted
to scale (1:24000) over DRG's and again check and edited. Then
HU coding was assigned as per National Instruction 170-304. The
data was then exported using the Grass command v.out.arc. It
was imported into ARC UTM-NAD83 projections and merged into this
one dataset called _________________.e00 for official release and
distribution. This e00 files contains _______________ and is in
a _________________ projection. Quality checks and edits were made
to ensure the data was still accurate at 1:24000 after conversion to
ARC e00 format.
While the HUA line work was being digitized the county lines and state
for Tennessee was also digitized from the DRG's. This data is available
from NRCS-TN also and meets the same standards as the HUA dataset.
* NOTE: Any use of trade, product, or firm names is for descriptive
purposes only and does not imply endorsement by the U.S. Government.
Although this metadata file is intended to document the data set in non-
proprietary form, some reference to various GIS licensed software products
may occur and is for reference only.
Dataset copied.
Polygon identifiers are manually assigned and checked with the
original base map using the attribute file.
The accuracy of this data is based on contour information derived
from the DRG's produced by the Tennessee Valley Authority. The DRG's
used to digitize the HU boundaries in this project were certified to
meet National Federal Map Accuracy Standards. HU boundary interpretations
may be supplemented with site visits or local knowledge of HU boundaries;
needed to determine unusual or difficult terrain areas or due to drainage
pattern changes by man's activities. The differences in positional accuracy
between the true hydrologic unit boundaries and their digitized map
locations is unknown.
A Hydrologic Unit (HU) is an area of land upstream from a specific
point on a stream (designated as the outlet). The HU boundary includes
all the source area that could contribute surface water runoff directly
and indirectly to the outlet. For the purposes of this effort, HU areas
(not always pure hydrologic units) are grouped together depending upon
their size. Areas grouped together in size units between 40,000 and
250,000 acres are referred to as watersheds and assigned an appropriate
eleven (11) digit code. The watersheds are then hydrologically subdivided
into sub-watersheds and assigned a fourteen (14) digit code. The sub-watersheds
typically range between 3,000 and 40,000 acres in size, but a small percentage
may fall outside this range for various local reasons.
Polygon and chain-node topology present. The tests of logical consistency
are performed using vendor software. All internal polygons were tested for
closure with vendor software and were checked on hard copy plots.
The U.S. Department of Agriculture, Natural Resources Conservation Service
should be acknowledged as the data source in products derived from these data.
The data should not be used at scales greater than 1:24,000.
This is public information and may be interpreted by organizations,
agencies, units of government, or others based on needs; however,
they are responsible for the appropriate application of the data.
Federal, State, or local regulatory bodies are not to reassign to the
Natural Resources Conservation Service any authority for the decisions
they make. The Natural Resources Conservation Service will not
perform any evaluations of these maps or purposes related solely
to State or local regulatory programs.
Photographic or digital enlargement of these maps to scales greater than
that at which they were originally delineated can cause misrepresentation
of the data. If enlarged, the maps will not include the fine detail that
would be appropriate for mapping at the small scale. Digital data files
are periodically updated. Files are dated, and users are responsible for
obtaining the latest version of this data.
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