Isogonic lines

Isogonic lines   -  Lines drawn across aeronautical charts to connect points having the same magnetic variation. The figure above illustrates  the isogonic lines that identify the number of degrees of variation in their area. The line that passes near Chicago is called the agonic line. Anywhere along this line the two poles are aligned, and there is no variation. East of this line, the magnetic pole is to the west of the geographic pole and a correction must be applied to a compass indication to get a true direction.