By convention, declination is positive when magnetic north is east of true north, and negative when it is to the west . " Isogonic lines " are lines on the Earth's surface along which the declination has the same constant value, and lines along which the declination is zero are called " agonic lines ".
परिभाषा
an imaginary line connecting points on the Earth''s surface where the magnetic declination is the same पर्याय: isogonal line, isogone,