Definition - What does Isogonic Line mean?
An Isogonic line is an imaginary line that depicts the earth’s magnetic declination. Isogonic lines are associated with magnetic declination and along these lines the earth’s magnetic declination remains constant. Wherever the magnetic declination along these isogonic lines is zero, those lines are given a slightly different name, i.e., agonic lines.
Petropedia explains Isogonic Line
Magnetic declination is defined as the angle of horizontal plane between magnetic north and true north. This angle changes with time as the earth rotates along its axis. The isogonic lines form the basis of isogonic maps or charts. Isogonic lines are the imaginary lines that join the points on the surface of the earth where the magnetic declination is constant or same.Over a long period of time, i.e., more than hundred years time frame, the magnetic declination tends to change by 2 to 2.5 degrees depending on the distance between the Earth’s magnetic poles.