Deviation is the difference between magnetic North and compass North. Unlike variation ( which is different at different places on earth), the deviation is magnetic influence of the immediate environment (SHIP) upon the compass. Every ship has certain induced magnetism whose magnitude may be affected by the type of cargo loaded and additional fixture installed on ship. The deviation depends on the orientation of the ship within the earth magnetic field. Practically, the compass deviation depends on the heading of the ship. This is because the RESULTANT magnitude of magnetic force on ship (which ultimately affects the magnetic compass), gets affected by ship's orientation with reference to the magnetic lines of force generated by earth's poles. This is the reason why a Deviation Table is posted on the bridge showing the deviation values for each heading of ship.
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