Isogonic line

An isogonic line is a line on a chart joining points of equal magnetic variation, drawn from the World Magnetic Model and updated each five-year epoch. Where variation is zero the line is the agonic line, separating easterly from westerly variation. Navigators read variation from the chart compass rose rather than off isogonic lines directly, but the lines show the regional pattern and rate of change. They differ from isoclinic lines, which join equal magnetic dip, and from the aclinic line marking zero dip at the magnetic equator. Variation feeds compass error: compass error equals variation plus deviation.