Deviation card

A deviation card is the table or curve listing the magnetic compass deviation for the ship’s head at intervals around the compass, usually every 15 or 45 degrees. The compass adjuster produces it after swinging the ship, signs it, and posts it at the steering position so the watchkeeper can convert between compass and magnetic course on any heading. Deviation is read off and applied east or west alongside variation to get compass error. The card is revalidated after structural changes, cargo affecting the magnetic signature, long periods on one heading, or a large change in magnetic latitude, and a deviation log records the checks.