Agonic Line

The specific isogonic line along which magnetic variation is zero, meaning that true north and magnetic north are aligned and no correction is needed when converting between true and magnetic courses. In the contiguous United States, the agonic line runs roughly from the Great Lakes region southward through the eastern states. West of this line, magnetic variation is easterly; east of it, variation is westerly. The agonic line slowly shifts over time as the Earth's magnetic field changes.