Plain English Meaning

Lines on a chart that link all the places where the compass points off from true north by the same amount and in the same direction.

Full Definition

Lines drawn on aeronautical and navigational charts that connect points of equal magnetic variation — that is, points where the angular difference between true north and magnetic north is the same. Each isogonic line is labeled with that variation in degrees east or west.

Context Anchor

Seen in discussions of magnetic variation, especially when comparing true directions with magnetic compass directions.

Derivation

From Greek 'iso-' meaning 'equal' and 'gonia' meaning 'angle.' So an isogonic line is literally an 'equal-angle line' — every point on it shares the same angular difference between true and magnetic north.