Magnetic North and True North are not the same.
Magnetic North is what a compass indicates by aligning with the Earth’s magnetic field lines.
True North is the direction of the geographic North Pole of the earth, as indicated by the meridians.
The angle difference between Magnetic North and True North is based on the compass’s location on Earth and changes over time. Those that travel long distances are required to make subtle course corrections based on their location due to the Magnetic Declination variances. Declination maps must be consulted in order to calculate the correct Magnetic Declination to add or subtract from a magnetic compass’s reading.
Magnetic Declination is not a factor for divers because we typically travel comparatively short distances while underwater, and do not use navigational charts to plan our route.
Magnetic Deviation of a compass is caused by the nearby presence of metal objects and electric circuits that produce local magnetic fields.
The resulting indication of North is called Compass North.
Boat and aircraft compasses suffer a large amounts of magnetic deviation due to the engines, additional electronics and in the case of boats, iron hull construction. To compensate for magnetic deviation, technicians determine the angle between Magnetic North and Compass North using specialized tools and correct for it by precisely placing magnetic compensators close to the compass.
Magnetic variation will affect a diver’s compass if it is worn or placed near metal dive watches, some dive computers or diver’s tools.
Diving near shipwrecks will also cause magnetic deviation in diver’s compasses. Electronic means of navigation, such a GPS, are not affected by magnetic deviation, and are often used to determine the magnetic deviation of a compass.