Abstract: Method and apparatus are disclosed for compensating a magnetic compass in a vehicle for magnetic deviation. With the compass installed, the angle indicated by the magnetic compass relative to a direction axis of the vehicle is measured. A test magnetic field is added at a known angle relative to the direction axis of the vehicle. The strength of the test magnetic field required to displace the direction indicated by the compass through a predetermined angle is memorized. Then, the strength of the resultant magnetic field is calculated using the measured angle, the predetermined angle and the memorized strength of the magnetic field. After determining the strength of the resultant magnetic field with the vehicle oriented in any selected direction, the vehicle is then reoriented in a direction one hundred eighty degrees away from the selected direction and the strength of the resultant magnetic field in that orientation is determined in the same way.