Deviation, Magnetic

the deviation of the movable system of a compass from the position fixing the direction toward the earth’s magnetic pole (in the case of a magnetic compass) or to the geographical pole (in the case of a gyrocompass). The deviation of a ship’s magnetic compass is due to the action of local magnetized bodies (for example, the steel hull of a boat) and the electromagnetic fields of ships’ electrical and radio equipment. Deviation is eliminated by means of auxiliary magnets that create a system of compensating magnetic fields. Corrections for residual deviation are made using tables. A gyrocompass is free from magnetic deviation, but its readings are affected by changes in course, speed, and geographical latitude of the position (velocity deviation); motion with acceleration (ballistic deviation); and rolling, particularly with northeast and northwest courses (quadrantal deviation). Deviation of a gyrocompass is limited or compensated for by correctors.