A map showing the isogonic lines. On such a map each line is
characterized and marked with the degrees and direction of variation of
the compass upon itself.

Synonym–Declination Map.

[Transcriber's note: The file Earth_Declination_1590_1990.gif provided
by the U.S. Geological Survey (http://www.usgs.gov) is an animation of
the declination of the entire earth.]