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.
Plain English
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.
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.
Why Pilots Care
Allows pilots to apply the correct variation value when converting between true and magnetic headings during navigation.
Grounding Statement
If two airports lie on the same isogonic line, they have the same magnetic variation.
Intuition Check
Do not read an isogonic line as a route, airway, or boundary. It only shows equal magnetic variation across the map.
Example Sentence 1
While planning the cross-country, she noted the route crossed the 6°W isogonic line, so she applied 6 degrees of westerly variation to her true course.
Example Sentence 2
Isogonic lines on the chart showed that variation increased steadily as the flight progressed eastward.