2004. United States. Forest Service. This polygon shapefile is commonly called Bailey's ecoregions and shows ecosystems of regional extent in the United States, Puerto Rico, and the U.... National Atlas of the United States.
2003. United States. Environmental Protection Agency. This polygon shapefile shows Omernik's Level III ecoregions, derived from a 1:7,500,000 map created by J.M. Omernik in 1987 and from refinements of... National Atlas of the United States.