areas that exhibit different ecological zones based on elevation. Citation: National Atlas of the United States. Omernik ecoregions Level 3: 3: This map layer 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 Omernik's framework that were made for other projects. Ecoregions describe areas of general similarity in ecosystems and in the type, quality, and quantity of environmental resources. Omernik's ecoregions are based on the premise that a hierarchy of ecological regions can be identified through the analysis of the patterns and the composition of both living and nonliving phenomena, such as geology, physiography, vegetation, climate, soils, land use, wildlife, and hydrology, that affect or reflect differences in ecosystem quality and integrity. All the characteristics are considered when determining ecoregions, but the relative importance of each characteristic may vary from one ecoregion to another. Level III is the most detailed level available nationally for this system of ecoregions. Citation: National Atlas of the United States. Geology: This data set contains boundaries for major geologic units in the United States. The data depict the geology of the bedrock that lies at or near the land surface, but not the distribution of surficial materials such as soils, alluvium, and glacial deposits. This is a revised version of the April 2004 data set. The data are generalized from a compilation prepared for use in the Geologic Map of North America, to be published in hard copy by the Geological Society of America and released as a digital file by the U.S. Geological Survey. Citation: National Atlas of the United States. Regional Watersheds w/ Legend: Watersheds w/ Legend: This map layer contains hydrologic unit boundaries and codes for the United States. It was revised for inclusion in the National Atlas of the United States of America, and updated to match the streams file created by the USGS National Mapping Division (NMD) for the National Atlas of the United States of America. This is a revised version of the November 2002 map layer. Citation: National Atlas of the United States. Subregional Watersheds Filled: Watersheds Filled: This map layer contains hydrologic unit boundaries and codes for the United States. It was revised for inclusion in the National Atlas of the United States of America, and updated to match the streams file created by the USGS National Mapping Division (NMD) for the National Atlas of the United States of America. This is a revised version of the November 2002 map layer. Citation: National Atlas of the United States. NEON Domains: Domains: NEON Domain’s located within the boundary of the 48 contiguous states. PRISM Avg Min Temp 1971-2000: Avg Min Temp 1971-2000: This map layer contains spatially gridded average annual minimum temperature for the climatological period 1971-2000. Distribution of the point measurements to a spatial grid was accomplished using the PRISM model, developed and applied by Chris Daly of the PRISM Climate Group at Oregon State University. Citation: PRISM Climate Group, Oregon State University, http://prism.oregonstate.edu.