Plain English Meaning

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.

Full 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.

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 to determine the exact magnetic variation needed to convert between true and during .