Variation (magnetic)

Variation, also called declination, is the horizontal angle between true north and magnetic north at a given position, measured east or west. It is the magnetic component of compass error that depends only on location and date, not on the ship, distinguishing it from deviation. Chart compass roses give variation for a stated year with an annual change rate, and the World Magnetic Model and isogonic lines map its global pattern. The navigator applies it with deviation to convert between true and compass: compass error equals variation plus deviation, with easterly added and westerly subtracted going from compass to true.