Air Force Manual (AFMAN) 15-111 Practice Test

Magnetic direction is measured relative to magnetic north, while true direction is measured relative to true north. The difference between true north and magnetic north is variation. If variation is easterly, magnetic north lies to the east of true north, so the magnetic bearing to a given direction is smaller than the true bearing by that amount, meaning you subtract the easterly variation. If variation is westerly, magnetic north lies to the west of true north, so the magnetic bearing is larger than the true bearing by that amount, meaning you add the westerly variation. So, when converting from true to magnetic, you add the westerly variation and subtract the easterly variation. For example, true direction 045° with 12° east variation gives a magnetic direction of 033°; with 12° west variation it becomes 057°.