Definition - What does Isogonic Chart mean?
An Isogonic Chart is a graph that depicts the isogonic lines on the earth. Isogonic lines are lines that depict the earth’s magnetic declination and along these lines, the earth’s magnetic declination remains constant. Thus the lines shown on isogonic chart show the areas where the magnetic declination is 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 Chart
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.
While taking a glance of the isogonic chart, a reader can understand that all the locations where the isogonic lines are passing through remains unaffected by the magnetic declination, i.e., the magentic declination is constant.