Magnetic Bearing

A magnetic bearing is the direction to an object measured clockwise from magnetic north. It differs from a true bearing by the amount of variation at the observer's position and from a compass bearing by the deviation of the magnetic compass on the heading being steered. Bowditch (American Practical Navigator) places the magnetic bearing as the intermediate step in the correction chain: compass bearing plus deviation gives magnetic bearing; magnetic bearing plus variation gives true bearing. This convention is universally applied and memorised using the mnemonic CADET. Magnetic bearings are quoted in practice when communicating between the magnetic compass and the chart plotter before the variation correction has been applied, or when working in areas where the chart is graduated in magnetic degrees—increasingly rare but still encountered on older Admiralty charts and regional publications. The OOW must clearly identify whether a bearing is true or magnetic when recording it, to avoid a systematic plotting error. In high-latitude areas where variation may exceed 30°, failing to distinguish magnetic from true bearings has caused grounding accidents, emphasising that precise terminology in bearing notation is a safety-critical discipline.