Definition
A line drawn on an aeronautical chart connecting points that have the same magnetic variation — that is, the same angular difference between true north and magnetic north.
Plain English
A line on a chart that links all the places where the compass points the same amount off from true north.
Context Anchor
Seen on aeronautical charts and in discussions of compass use, navigation, and magnetic variation.
Derivation
From Greek 'iso-' meaning 'equal' and 'gonia' meaning 'angle.' So an isogonic line is literally a 'line of equal angle' — every point on it shares the same magnetic variation angle.
Why Pilots Care
Gives the exact variation value needed to convert a true heading or course into the magnetic heading actually flown on the compass.
Analogy
An isogonic line is like a contour line on a terrain map. A contour line connects places with the same height; an isogonic line connects places with the same magnetic variation.
Intuition Check
An isogonic line is not a route to fly. It is a chart line showing equal magnetic variation.
Example Sentence 1
While planning the cross-country, she noted the isogonic line along her route showed 7° west variation and corrected her true course accordingly.
Example Sentence 2
Following the isogonic lines across the chart helped the crew maintain accurate magnetic courses for the entire leg.