The Geographic Information Retrieval and Analysis System (GIRAS) <br /> was developed in the mid 70s to put into digital form
a number <br /> of data layers which were of interest to the USGS. One of these <br /> data layers was the Hydrologic Units.
The map is based on the <br /> Hydrologic Unit Maps published by the U.S. Geological Survey <br /> Office of Water Data Coordination,
together with the list <br /> descriptions and name of region, subregion, accounting units, and <br /> cataloging unit. The
hydrologic units are encoded with an eight- <br /> digit number that indicates the hydrologic region (first two digits), <br
/> hydrologic subregion (second two digits), accounting unit (third two <br /> digits), and cataloging unit (fourth two digits).
<br /> <br /> The data produced by GIRAS was originally collected at a scale of <br /> 1:250K. Some areas, notably major cities
in the west, were <br /> recompiled at a scale of 1:100K. In order to join the data together <br /> and use the data in a
geographic information system (GIS) the data <br /> were processed in the ARC/INFO GUS software package. Within the <br />
GIS, the data were edgematched and the neatline boundaries between <br /> maps were removed to create a single data set for
the conterminous <br /> United States. <br /> <br /> NOTE: A version of this data theme that is more throughly <br /> checked
(though based on smaller-scale maps) is available here: <br /> http://water.usgs.gov/lookup/getspatial?huc2m <br /> <br />
HUC, GIRAS, Hydrologic Units, 1:250