Deviation card

A reference chart used on a vessel to indicate the compass error caused by local magnetic influences, showing the necessary corrections for different headings.

A deviation card is an essential tool in marine navigation, particularly when using a magnetic compass. It accounts for the magnetic deviation, which is the error introduced by the vessel’s own magnetic field or other local magnetic influences, such as electrical equipment or metal structures on board. This deviation is specific to each vessel and can vary depending on the compass heading. The deviation card lists the amount of deviation for various compass headings, allowing the navigator to apply the necessary correction to the compass reading to obtain an accurate course. 

Deviation is different from variation, which is the difference between true north and magnetic north, and is caused by the Earth’s magnetic field. While variation is constant for a given location and can be found on nautical charts, deviation is unique to each vessel and must be determined through a process called compass swinging. During this process, the vessel is sailed on different headings, and the actual compass readings are compared to the known true headings. The resulting deviation values are then recorded on the deviation card. By using this card, mariners can ensure that their compass readings are as accurate as possible, improving the safety and precision of their navigation.