Isogonic Line

A line drawn on an aeronautical chart connecting points that have the same magnetic variation (declination). Isogonic lines are shown as dashed magenta lines on sectional charts, labeled with the variation value (e.g., "10°W"). Pilots use them during flight planning to determine the local magnetic variation and convert between true and magnetic courses. The variation changes gradually across geographic areas, so the isogonic line nearest the route of flight provides the best correction value.