Chart of the curves of equal magnetic variation, 1858 / by Frederick J. Evans, Master, R.N. - Maps - Huntington Digital Library
South Polar chart shewing the discoveries and track of H.M.S. Erebus and Terror during the years 1840. 1. 2. 3 with the lines of equal magnetic dip and variation and the position
MAGNETIC VARIATION
Historical main field change and declination
Earth's Magnetic Declination - Science On a Sphere
Magnetic Declination Varies Considerably Across The United States | U.S. Geological Survey