isogonic chart

magnetic chart

A chart showing the distribution of one of the magnetic elements, such as by isogonic lines. Also called an isogonic chart. See isogonic lines.
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The book began as an academic dissertation, and the author's principal interest lies in such maritime arcana as isogonic charts of compass variation and such ship-shape topics as nautical pennants.
Halley invented isogonic charts and was the first to systematically study the Earth's magnetic field and its variation.
Isogonic charts show the variation and annual change and are published every five years.