Since they emanate from north and south poles, the needle will align itself in the north-south direction. Unfortunately the magnetic poles are not situated in the same place as the true poles and thus an error occurs in the true reading of the compass. Therefore, before compass readings can be used for navigation they must be corrected for such errors. This is the name given to the error caused by the difference in position between the true and magnetic poles. As its name denotes, it varies from place to place across the world, but it is accurately tabulated for the navigator's use.
Variation is the error in the compass caused by the Earth's magnetism.
It is always named E or W according to which direction the card is deflected away from true north. The second of the two errors which affect the magnetic compass, deviation, is caused by the magnetic influence of anything near the compass needle. Someone placing a metal knife alongside the binnacle, for example, will cause a deflection of the compass needle and result in deviation error. Steel in the construction of the ship, electric circuits, motors, and so on, can all affect the compass and create a deviation error. Deviation is the error in the compass caused by the ship's magnetism.