A bearing or line on the chart that, when maintained, keeps a vessel clear of a specific hazard such as a shoal or reef.
In practice
The OOW plots clearing lines on the chart and sets corresponding bearing markers on the radar or ECDIS before entering coastal or pilotage waters. If a light or conspicuous beacon opens or closes on a clearing bearing, the vessel's position relative to the hazard changes and immediate action may be required. Clearing lines are particularly valuable in reduced visibility when visual confirmation is impossible, as radar bearings of charted objects can be substituted. Their robustness against compass error—requiring only a relative relationship rather than an absolute position—makes them complementary to GPS and ECDIS position monitoring.
Regulatory detail & full definition
A clearing line is a bearing or line on the chart that, when observed or maintained by the vessel, ensures it remains clear of a specific hazard—a reef, shoal, wreck, or rock. Unlike a leading line, which directs the vessel along a required track, a clearing line simply marks a boundary: as long as a charted object bears greater or less than a specified angle, the vessel is safe. Clearing lines are derived from the geometry of charted hazards and plotted during passage planning. The Admiralty Manual of Navigation discusses clearing bearings and clearing angles as standard pilotage tools.
The OOW plots clearing lines on the chart and sets corresponding bearing markers on the radar or ECDIS before entering coastal or pilotage waters. If a light or conspicuous beacon opens or closes on a clearing bearing, the vessel's position relative to the hazard changes and immediate action may be required. Clearing lines are particularly valuable in reduced visibility when visual confirmation is impossible, as radar bearings of charted objects can be substituted. Their robustness against compass error—requiring only a relative relationship rather than an absolute position—makes them complementary to GPS and ECDIS position monitoring.