Definitions for "Isogonic line"
In the study of terrestrial magnetism, a line drawn through all points on the earth's surface having the same magnetic declination; not to be confused with magnetic meridian. The particular isogonic line drawn through all points having zero declination is called the agonic line. Compare isoclinic line, isogon.
A line of constant magnetic variation.
an imaginary line connecting points on the Earth's surface where the magnetic declination is the same