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2009. World Wildlife Fund (WWF). This raster layer is a void-filled digital elevation model (DEM) of North America at 15 arc-seconds resolution. Elevation is expressed in meters an... U.S. Geological Survey.
2005. National Atlas of the United States. The Topographic and Bathymetric Shaded Relief of North America map layer shows depth and elevation ranges using colors, with relief enhanced by sha... National Atlas of the United States.