The basic premise of compass adjusting (that is compensation aboard ship for magnetic fields caused by the vessel itself and the determination of residual deviation) is that a reference direction must be available during the operation. Normally visual ranges or the azimuth of a celestial body—usually the Sun—are used, but in some areas and climates reduced visibility often does not permit the use of either. Compass adjusters are frequently driven in desperation to use gyro-compass, radar, or radio bearings—with highly speculative results as the errors of these instruments are generally unknown, and fairly large compared to the accuracy required of the magnetic standard compass (± ½°) which is the primary direction standard aboard ship.