NORTH and true south, that is earth's true magnetic north pole and true magnetic south pole. It is established by astronomical observations.Magnetic meridian:It is the DIRECTION shown by a freely floating and balanced magnetic needle free from all other attractive forces. It is established by magnetic poles.Grid meridian:For a survey of a state, the true meridian of the CENTRAL place is TAKEN as a reference meridian for the whole state and is called grid meridian.Arbitrary meridian:It is the meridian taken in any convenient direction towards a permanent and prominent mark or signal. It is used to determine relative DIRECTIONS of various lines in a small traverse or small area.