agonic line

noun

1. an imaginary line on the surface of the earth connecting all points at which the declination of the magnetic field of the earth is zero.

noun

1. an imaginary line on the surface of the earth connecting points of zero magnetic declination

1. An imaginary line on the Earth’s surface connecting points where the magnetic declination is zero. The agonic line is a line of longitude on which a compass will show true north, since where magnetic declination is zero, magnetic north coincides with geographic north.