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The North America elevation data were derived from the GTOPO30 elevation dataset (http://eros.usgs.gov/products/elevation/gtopo30.html) produced by the U.S. Geological Survey and from the Canada3D elevation dataset produced by the Centre for Topographic Information (Sherbrooke), Natural Resources Canada. GTOPO30 is a global digital elevation model (DEM) with a horizontal grid spacing of 30 arc seconds (approximately 1 kilometer). GTOPO30 was derived from several raster and vector sources of topographic information. It was developed between 1993 and 1996 through a collaborative effort led by staff at the U.S. Geol The North America elevation data were derived from the...