Definition
An imaginary line on the Earth's surface connecting points where magnetic north and true north are aligned, so that magnetic variation is zero. A compass on the agonic line points to true north without any easterly or westerly variation correction.
Plain English
A line on the map showing where your compass happens to point exactly to true north, with no east or west error to correct for.
Context Anchor
Seen in compass, navigation, and chart discussions when comparing magnetic direction with true direction.
Derivation
From Greek 'a-' meaning 'without' and 'gonia' meaning 'angle.' Literally 'no angle' — there is no angle between true north and magnetic north along this line. This contrasts with isogonic lines, which connect points of equal (but non-zero) variation.
Why Pilots Care
On or near the agonic line, no variation correction is needed when converting between true and magnetic headings. Everywhere else, pilots must apply easterly or westerly variation, and forgetting to do so produces navigation errors.
Intuition Check
Do not read “line” as a painted or visible line on the ground. An agonic line is an invisible map line showing where the compass direction and true north direction match.
Example Sentence 1
Because their flight crossed the agonic line, the pilot noted that true and magnetic headings matched along that segment of the route.
Example Sentence 2
As the agonic line slowly shifts over decades, updated charts reflect the new location where no magnetic variation applies.