Two charted objects in line that define a safe course into or within a port, keeping the vessel clear of hazards.
Quick facts
Regulation
SOLAS Chapter V
In practice
The OOW approaching a port with published leading lines should identify them on the chart before arrival, note the range at which they become visible, and monitor the alignment continuously during the approach. If the vessel drifts off the leading line, immediate helm action is required. SOLAS Chapter V, Regulation 34 on safe navigation requires that the passage plan include all leading lines relevant to port approaches. During pilotage, the pilot and OOW jointly monitor leading-line transits as a primary position reference, with radar and ECDIS as secondary confirmation.
Regulatory detail & full definition
A leading line is a straight line defined by two charted objects—typically a pair of lighthouses, beacons, or distinctive shore features—that lie in transit and mark the axis of a safe channel into or out of a port or anchorage. When the navigator keeps both marks in line, one directly behind the other, the vessel is on the defined safe track. Leading lines are designed by hydrographers to route vessels clear of shoals, rocks, and other hazards, and are shown on charts with a bearing and often lit at night for 24-hour use. The Admiralty Manual of Navigation describes the use of leading lights as among the most reliable pilotage aids available.
The OOW approaching a port with published leading lines should identify them on the chart before arrival, note the range at which they become visible, and monitor the alignment continuously during the approach. If the vessel drifts off the leading line, immediate helm action is required. SOLAS Chapter V, Regulation 34 on safe navigation requires that the passage plan include all leading lines relevant to port approaches. During pilotage, the pilot and OOW jointly monitor leading-line transits as a primary position reference, with radar and ECDIS as secondary confirmation.