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.
It is always named E or W according to the direction the card is deflected from true north. This is easy. On every chart there are a number of compass roses as described in next paragraph below. In the centre of each compass rose is listed the variation for that area and the amount it is likely to change in one year, which is usually fairly small. Since most navigation on a chart involves the use of the compass, a reproduction of a compass card is printed at strategic points across the face of every chart.