Data from the protected and critical watersheds layer for the
state was used during the development of the HU boundaries.
This coverage supersedes data on past versions, particularly
the cama24.hu coverage, which was completed as a preliminary
to this statewide version. Some boundaries may have changed
between the two versions.
There are two additional coverages which were generated from
this data, the huncrb-river basins; and huncsb-subbasins files.
See the cross references section for details.
The following is an excerpt from the "North Carolina Hydrologic
Unit River Basin Study," USDA, Natural Resources Conservation
Service, November 1995.
Under the sponsorship of the Water Resources Council,
a nationally uniform hydrologic unit system was developed in
1974 by the US Geological Survey's Office of Water Data
Coordination. This system divides the country into 21 regions,
222 subregions, 352 accounting units, and 2,149 cataloging units,
based on surface hydrologic features. A hierarchical code
consisting of two digits for each of the above four levels
combine to form an eight-digit hydrologic unit. The hydrologic unit
system is used to identify any hydrologic area of interest. An
eight-digit hydrologic unit generally covers 700 or more square
miles.
In 1978, the USDA-Natural Resources Conservation Service (NRCS)
issued a policy that all resource inventories and surveys were
to be coded with and capable of being retrieved by hydrologic
unit codes (HUCs). At about the same time, NRCS initiated a
national program to further subdivide HUCs into watershed-sized
areas (nominally 250,000 acres, or 390 square miles) for use in
water resource planning. An extension of three digits was added
to the eight-digit HUC to designate sub-watersheds, thus forming
elevin-digit HUCs.
This 1995 Hydrologic Unit (HU) Study divides the state river basins
and subbasins into smaller fourteen-digit hydrologic units
that will be useful in targeting project activities, resource
inventories, and reporting conservation activities. These
fourteen-digit hydrologic units of approximately 4,000 acres
(6 sqare miles) to 50,000 acres (78 square miles) are small enough
in size to be useful as a planning and reporting tool for the
USDA-Natural Resources Conservation Service (NRCS) and other
government and state agencies.
This 1995 fourteen-digit HU map will replace the 1974 eight-
digit HU map [U.S. Water Resource Council (WRC)] and the 1978
NRCS eleven-digit HU map as the locating tool for planning in
NRCS. The 1995 HU map were produced using 7.5 minute USGS
quadrangle base maps (1:24,000 scale).
14-digit number breakdown:
Assigned by US Water Subunit Reporting Unit (established
Resource Council (1978 NRCS) during this study)
|---------------------|----------|--------|
0 3 0 6 0 1 0 1 0 1 0 0 1 0
|----|----|-----|-----|
| | | |
| | | Cataloging Unit Boundary
| | Accounting Unit Boundary
| Subregional Boundary
Regional Boundary
NOTE: The subbasin 6-digit code (SUBBASIN#) was developed independently
from the hydrologic unit codes, from an entirely different numbering
system. (Example of code: 03-03-01.) The first two numbers identify the
river basin designations adopted by the Environmental Protection Agency.
(Date of source: 1974.) Subbasin boundaries were added within individual
river basin boundaries (by the office that is now known as the Division
of Water Quality, Water Quality Planning Section). Sequential numbers
(example -01) were added by DWQ to the first four-digit EPA code to give
each subbasin a unique six-digit identifier. (Each subbasin incorporates
three or more hydrologic units. The subbasin boundaries have been
modified to be coincident with the hydrologic unit boundaries.)
Contact Alan Clark at DWQ for more information on the subbasin# code
(733-5083, extension 570).
system filename: huncrb file size = 0.8 mb
(huncrb and huncsb need to be updated whenever hunc is
updated).
Revisions and updates to this layer include:
3.) filename: huncrb1298 The December 1998 update to this layer
consisted of projecting the data from NAD27 datum, State Plane
projection, units of measure feet TO: NAD83 DATUM, State Plane
PROJECTION, UNITS OF MEASURE METERS. This was done to comply with
the NC Geographic Information Coordinating Council's "Statement of
Direction for North Carolina Corporate Geographic Database
Horizontal Reference, Datum and Unit of Measure". This reprojecting
was done in various ways depending on the data type and content.
Vector data was projected using the 'project' command in ESRI's Arc
software and topology was cleaned and built based on coverage needs.
Raster data was projected using ESRI's Grid module and various steps
as applicable.
2.) filename: huncrb496 The April 15, 1996 update replaced
the old nc.rivbasin file which was a general reference layer only.
It is archived under the nc.rivbasin name. The huncrb496 was
created in late 1996, but backdated to reflect the data currency
date of the Hydologic Units file.
1.) filename: rivbasin89 (previously called nc.rivbasin)
This file was created in the late 80s to assist in
identifying the major river basins in North Carolina. This
data was for display purposes only, ie. locator maps, since
its scale is 1:1,000,000. This file contains ABRV (CHO)
river basin name abbreviation; and, NAME (CHOWAN) attributes
in the PAT. The AAT contains system default attributes only.
This file was replaced with the huncrb river basins file
from the hydrologic (hunc) units file boundaries at 1:24,000-scale.