Title: Hydrologic Units at 1:250,000, 8-digit by hydrologic unit region
Originator: United States Geologic Survey
Abstract:
The Geographic Information Retrieval and Analysis System
(GIRAS) was developed in the mid 1970s to put into digital form a number of data
layers which were of interest to the USGS. One of these data layers was the
Hydrologic Units. The map is based on the Hydrologic Unit Maps published by the
U.S. Geological Survey Office of Water Data Coordination, together with the
list descriptions and name of region, subregion, basin, and sub-basin units.
The hydrologic units are encoded with an eight-digit number that indicates the
hydrologic region (first two digits), hydrologic subregion (second two digits),
basin unit (third two digits), and sub-basin unit (fourth two digits).
The data produced by GIRAS was originally collected at a scale of 1:250K. Some
areas, notably major cities in the west, were recompiled at a scale of 1:100K.
To join the data together and use the data in a geographic information
system (GIS), the data were processed in the ARC/INFO GIS software package.
Within the GIS, the data were edgematched and the neatline boundaries between
maps were removed to create a single data set for the conterminous United
States.
In the fall of 2002, the USDA-NRCS National Cartography and Geospatial
Center (NCGC) updated the attribute table by appending the name fields for the
2, 4, 6, 8-digit HU's from the 1:2M USGS 8-digit dataset. NCGC also made
attribute fields for 2, 4, 6, 8-digit hydrologic unit codes. Lastly, NCGC
created fields for "Acres", "HU_8_state", "Fips_C " (HUC2 state) and "Square Miles". The
"Acres" and "Square Miles" fields were calculated from the area field while the dataset had a projection
of Albers Equal Area. The dataset was then projected into Geographic, Deciamal Degrees,
NAD83.
Format: vector lines – Arc shapefile
Spatial Reference Information: Decimal Degrees, North American Datum 1983
