2003. USGS National Center for EROS. The North America Shaded Relief data were derived from the GTOPO30 elevation data. GTOPO30 is a global digital elevation model (DEM) with a horizon... National Atlas of the United States.
    
  
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      2003. Oak Ridge National Laboratory. The LandScan 2003 global population data set is a worldwide population database compiled on a 30" X 30" latitude/longitude grid. Census counts (at ... Oak Ridge National Laboratory.
    
  
    
      2003. National Geophysical and Solar-Terrestrial Data Center. This map shows light sources from human settlements around the world observed during 2003. The Defense Meteorological Satellite Program (DMSP) Oper... National Geophysical and Solar-Terrestrial Data Center.
    
  
    
      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.