What does deviation refer to in navigation?

Deviation in navigation specifically refers to the angular displacement of a ship’s magnetic compass due to local magnetic fields, primarily caused by the ship's own metallic structures and electrical equipment. When a vessel is in operation, these magnetic influences can distort the reading of the compass, resulting in what is known as deviation.

It's important to differentiate this from other terms in navigation. For instance, the distance from the North Pole does not directly relate to how the compass behaves under various influences, and chart-related errors pertain more to discrepancies between a charted course and an actual course taken rather than compass readings. The effect of surrounding magnetic fields on compass readings is more generally considered as variation, which involves the difference between true north and magnetic north. Deviation is specifically about the ship's own influence on its compass.

Understanding deviation is crucial for navigators to correct compass readings appropriately, ensuring accurate course plotting while at sea.