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

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.