Hydrologic Units Maps of the Conterminous United States for the EPA Clean Air Mapping and Analysis Program (C-MAP)

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 edge matched and the neatline boundaries between
maps were removed to create a single data set for the conterminous United
States.

This data set was compiled originally to provide the National Water Quality
Assessment (NAWQA) study units with an intermediate- scale river basin boundary
for extracting other GIS data layers. The data can also be used for
illustration purposes at intermediate or small scales (1:250,000 to 1:2
million).

[Summary provided by EPA]