2010. National Geophysical Data Center. This map layer shows isogonic lines (lines of constant declination of the Earth's magnetic field), derived from the International Geomagnetic Refer... National Atlas of the United States.
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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.
1849. Harvard Map Collection, Harvard College Library, Harvard University. Library. Open Collections Program, Duperrey, Louis-Isidore, 1786-1865., and Meyer, Joseph, 1796-1856. This layer is a georeferenced raster image of the historic paper map entitled: Wachsende Karte der Magnetischen Meridiane und Parallelen : Gegrunde... Harvard Map Collection, Harvard College Library.