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2010. National Geophysical Data Center. This map layer shows isoporic lines (lines of equal annual change, or secular variation, in the declination of the Earth's magnetic field), derived... National Atlas of the United States.
2010. National Geophysical Data Center. This line shapefile shows lines of equal annual change (secular variation) in the horizontal component of the total field intensity of the Earth's ... National Atlas of the United States.
2010. National Geophysical Data Center. This shapefile shows lines of equal annual change (secular variation) in the inclination of the Earth's magnetic field, derived from the Internatio... National Atlas of the United States.
2010. National Geophysical Data Center. This line shapefile shows lines of equal annual change (secular variation), in the total field intensity of the Earth's magnetic field, derived fro... National Atlas of the United States.
2010. National Geophysical Data Center. This line shapefile shows lines of equal annual change (secular variation) in the vertical component of the total field intensity of the Earth's ma... National Atlas of the United States.
2010. National Geophysical Data Center. This shapefile contains isodynamic lines (lines of equal total intensity of the Earth's magnetic field), derived from the International Geomagnetic... National Atlas of the United States.
2010. National Geophysical Data Center. This line shapefile shows lines of equal vertical intensity of the Earth's magnetic field, derived from the International Geomagnetic Reference Fie... National Atlas of the United States.
2002, 2003, and 2004. Geological Survey (U.S.). This point shapefile portrays major dams of the United States, including Puerto Rico and the U.S. Virgin Islands. The map layer was created by extr... Geological Survey (U.S.).
2006. Rawson, Peg. This point shapefile depicts major dams in the United States, including Puerto Rico and the U.S. Virgin Islands. The map layer was created by extra... United States. Bureau of Transportation Statistics.
2013. Waterborne Commerce Statistics Center (U.S.) and United States. Bureau of Transportation Statisticsf. This point shapefile depicts ports politically defined by port limits or USACE projects, excluding non-USACE projects not authorized for publicatio... United States. Bureau of Transportation Statistics.