1. Omernik's Level III Ecoregions of the Continental United States, 2003 Polygon Stanford University Public content 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.