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A dashed line joining points of equal magnetic variation.
Also known as: Magnetic variation line, Isogonal
Isogonic lines join places where magnetic variation is the same, and are printed on aeronautical charts with the year of measurement. Magnetic variation drifts, so the lines are dated and a chart's usefulness for navigation expires.
Joins points of equal magnetic variation on a chart, printed as a dashed line with its value.
Contexts: Navigation, Maps
Carries an epoch date because the magnetic field drifts, so the printed values are correct only for the stated year.
Contexts: Navigation, Maps
A long dashed line curving across a chart with a numeral and a sign printed in a break along its length.