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 Centr 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. Geological Survey's Earth Resources Observation and Science Center (EROS). In the United States, GTOPO30 data were derived from the National Elevation Dataset (NED). In Mexico, GTOPO30 data were derived from 3-arc-second elevation data from INEGI. These data were prepared by the U.S. Defense Mapping Agency (now National Geospatial-Intelligence Agency, NGA) using contour lines from the INEGI topographic national map series at 1:250,000 scale. Elsewhere, GTOPO30 data were derived from several sources.Canada3D was developed by Natural Resources Canada as a national-level elevation dataset at 30-arc-second resolution. Details on the derivation of Canada3D data are available at the Web link referenced below. In contrast to GTOPO30 data in Canada,Canada3D elevation data displayed relatively few artifacts and were of higher quality than GTOPO30, and were accordingly used for all land masses in Canada.
