U.S. Fish & Wildlife Service

Data Layer: HYDROLOGIC UNITS

You are invited to read the definitions of terms used in this data standard.

Name
 Hydrologic Unit Data
Description
 Digital maps originally created by the U.S. Geological Survey (USGS), and now being updated by the USDA Natural Resource Conservation Service (NRCS). The small-scale versions of these data are available on the Internet, free-of-charge. Both the 1:250,000- and 1:2,000,000-scale data sets are available nationally. The 1:24,000-scale data are not yet completed nationally, but status graphics are on the Internet, and further data updates are in progress. The 1:250,000- and 1:2,000,000-scale data are available in both Arc/Info and Spatial Data Transfer Standard (SDTS) formats.
Source
 USGS Hydrologic Units at 1:250,000- and 1:2,000,000-scale. The 1:250,000-scale data are available by regions or for the entire country.

NRCS Hydrologic Unit Boundaries at the 1:24,000-scale data. This site has pointers to the process for updating the data, contacts for obtaining the data, and a status map.

Reference(s)

Part 270, FW 6, Data Management and Standards, dated September 30, 2002.

USGS Water Resources - This site contains metadata, background documentation on the data, and text documents listing all the hydrologic unit codes in the United States. It also has pointers to the data site.
Use Instructions
 The hydrologic data layer produced by the USGS (or the NRCS 1:24,000-scale data layer) should be used wherever it is available in an appropriate scale. The hydrologic unit codes (attributes) referenced in the data sets are part of the Federal Information Processing Standards (FIPS), and are the nationally accepted identifiers for watersheds. Even if the mapped data are not available at the needed scale, the existing attribute coding scheme should be followed on any new maps created by the Service.

The 1:24,000-scale data set expands the codes to include 11- and 14-digit designations which will allow for better identification of the smaller watershed units. Hydrologic data are often used as a locational geographic information system (GIS) layer and form the basis for the Service's ecosystem unit boundaries. These data can be used to define the boundaries of a project area around a refuge or other unit.

Tools
The tools listed below may assist Service users in using the hydrologic data in GIS programs such as ArcView. The information available on the Internet is present in Arc/Info export format; however, some of the information does not import into ArcView in a very usable format due to the edited attributes in the tables. These files should simplify lookup functions.

Hydrologic data attribute table in a dBASE III format for downloading. This table combines the two-, four-. six-, and eight-digit codes (huc2, huc4, huc6, and huc) into one table that can be used to look up the descriptions from the other tables identified below. The file is compressed using PKZip. Depending on the browser, a right mouse-click may be needed to download the file. 

Two-, four-, six-, and eight-digit hydrologic unit codes and descriptions. Four files (n2.dbf, n4.dbf, n6.dbf, and n8.dbf) are compressed into one zipped file using PKZip. Depending on the browser, a right mouse-click may be needed to download the zipped file.
Phase
 Adopted
Approval Date
 March 21, 2000
Validation Date
 January 3, 2005
Data Steward
 Deb Southworth Green, National FWS GIS Coordinator, Branch of Data and System Services, Division of Information Resources and Technology Management

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These pages are maintained by the U.S. Fish and Wildlife Service, Division of Information Resources and Technology Management. Please feel free to contact Deb Southworth Green with any questions and comments.

Keywords=fish, wildlife, animals, birds, data, standards, layers, hydrologic, units
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