Bearing

In marine navigation, a bearing is the horizontal angle measured clockwise from a reference north direction to the line of sight toward a specific object or landmark, expressed in degrees from 000° to 360°. It must not be confused with vessel heading or course, which describe the direction the vessel itself is moving. Outside of navigation, the English homonym "bearing" denotes a mechanical machine element (such as a ball bearing); in nautical contexts, it strictly refers to directional angular measurements.

Bearings are classified according to the reference meridian used. True Bearing (TB) uses True Geographic North. Magnetic Bearing (MB) references Magnetic North and incorporates magnetic variation. Compass Bearing (CB) is read directly from a marine compass on a sailboat and must be corrected for both magnetic variation and compass deviation. Relative Bearing (RB) measures the angle clockwise from the vessel's bow (000° to 360°).

Courses of constant bearing, known as rhumb lines, were mapped as straight lines on Gerardus Mercator’s 1569 world map, revolutionizing chart work for mariners. In seamanship, taking frequent bearings is critical for collision avoidance: a constant bearing with a decreasing distance indicates a risk of collision. Sailors routinely combine visual hand-bearing compass readings with electronic positioning data obtained from GPS and electronic navigation devices to fix their position on nautical charts accurately.