
http://www.bts.gov/help/national_transportation_atlas_database.htmlhttp://www.bts.gov/help/national_transportation_atlas_database.html
http://gos2.geodata.gov/wps/portal/gos http://gos2.geodata.gov/wps/portal/gos
http://www.lib.virginia.edu/scholarslab/resources/gis/vagaz/index.htmlhttp://www.lib.virginia.edu/scholarslab/resources/gis/vagaz/index.html
http://www.personal.psu.edu/cab38/ColorBrewer/ColorBrewer_intro.htmlhttp://www.personal.psu.edu/cab38/ColorBrewer/ColorBrewer_intro.html