Isogonic Lines:

If the points on the globe that have similar magnetic declination at a point of time are joined, the imaginary lines so obtained are called isogonic lines.

Agonic Lines

These are imaginary lines constructed through joining the points at that the magnetic declination is zero, and therefore have the similar value of magnetic bearing and true bearings.

For reference to geodetic and other significant surveys, isogonic charts are published by agencies like Survey of India, on which isogonic and agonic lines are drawn on earth maps.