Magnetic Compass
A Magnetic Compass uses a magnetised needle or card that aligns with the magnetic field of the earth to indicate magnetic north. It is the oldest marine navigation instrument and, because it needs no power, remains a mandatory backup on every ship even in the age of GPS and the gyrocompass.
Its readings must be corrected for two errors: variation, the difference between magnetic and true north at a place, and deviation, the local error caused by the steel and equipment of the ship, recorded on a deviation card after the compass is adjusted.
When power fails and the gyro and electronics go dark, the magnetic compass still shows a heading — which is exactly why it is required by law as an independent backup. It is the last-resort heading reference that keeps a ship able to steer a course in any emergency.
What are variation and deviation?
Variation is the angle between magnetic and true north at a location; deviation is the compass error caused by the magnetism of the ship. Both are applied to convert a compass reading to a true heading.