Magnetic variation

Magnetic variation is the angle between true north and magnetic north at a place, named east or west according to which side of true north the compass needle points. It arises from the geometry of Earth’s main field and changes slowly with position and time, so charts print it inside the compass rose with the year of measurement and the annual rate of change. Variation is taken from the chart or the World Magnetic Model and combined with deviation to give compass error: compass error equals variation plus deviation. A navigator updates the rose figure to the current year before applying it.