Definition
Relating to or denoting a line on a chart that connects points of equal magnetic variation — that is, points where the angular difference between true north and magnetic north is the same. An isogonic line shows pilots how many degrees, and in which direction, magnetic north deviates from true north at any location along that line.
Plain English
Isogonic describes a line on a chart that joins all the places where the compass is 'off' from true north by the same amount. If you're anywhere along that line, the gap between true north and what your compass shows is identical.
Context Anchor
Seen in aeronautical chart legends and chart markings, especially when a chart explains magnetic variation.
Derivation
From the Greek 'isos' meaning equal, and 'gonia' meaning angle. Literally 'equal angle' — the line connects points where the magnetic variation angle is the same.
Why Pilots Care
Pilots consult isogonic lines to determine the exact magnetic variation needed to convert between true and magnetic headings during navigation.
Analogy
It is similar to a weather map line that connects places with the same temperature, except an isogonic line connects places with the same magnetic angle difference.
Intuition Check
Do not read isogonic as a route, airway, or boundary. It is chart information showing equal magnetic variation.
Example Sentence 1
While planning the cross-country, she checked the isogonic line crossing her route and applied 6° west variation to convert true course to magnetic course.
Example Sentence 2
Isogonic lines on the en route chart helped confirm the local magnetic variation before setting the heading.