1:250,000-scale Hydrologic Units of the United States

The Geographic Information Retrieval and Analysis System (GIRAS)
was developed in the mid 70s 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, accounting units, and
cataloging unit. The hydrologic units are encoded with an eight-
digit number that indicates the hydrologic region (first two digits),
hydrologic subregion (second two digits), accounting unit (third two
digits), and cataloging 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. In order to join the data together
and use the data in a geographic information system (GIS) the data
were processed in the ARC/INFO GUS 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.

HUC, GIRAS, Hydrologic Units, 1:250