Pilotage
What Is a Leading Line? Transits and Leading Marks Explained
Published · Last checked October 2026
A leading line is a charted line of safe water defined by two marks, beacons or lights, one behind the other. When you see them exactly in line (a transit), you are on the safe track. The rear mark is normally higher than the front so you can see both. If they drift apart, you have moved off the line and should steer towards the side the front mark has moved to. Charts show leading lines with their true bearing.
How a leading line works
Two fixed objects in line define a position line with no instruments at all. Harbour authorities build pairs of leading marks or lights to give a precise track through a narrow entrance or past dangers. On the chart, the line is drawn as a solid line where it is safe to follow, often labelled with the bearing, for example 'Ldg Lts 270°'.
Correcting off the line
If the marks separate, you are off the line. The rule of thumb is to steer towards the front mark: move to the side that the front, lower mark appears to have moved to.
| What you see | Where you are | What to do |
|---|---|---|
| Front and rear exactly in line | On the line | Hold your course, allowing for tide |
| Front mark left of the rear | Right of the line | Move left |
| Front mark right of the rear | Left of the line | Move right |
The further apart the two marks are, the more sensitive the line becomes as you close in. Expect small corrections to look bigger near the entrance.
Natural transits
You do not need purpose-built marks. Any two charted objects in line, such as a church spire and a headland, or a beacon and a hilltop, give a transit. Sailors use them to:
- Clear a danger: 'keep the spire open of the point' to stay off a rock.
- Check compass deviation by comparing your heading along a known transit.
- Watch whether the anchor is dragging, by noting two objects in line ashore.
- Fix your position when two transits cross.
At night
Leading lights work the same way: front lower, rear higher. They often have the same colour and different rhythms, or one is fixed and the other occulting. Some approaches use a single sectored light instead. Plan both in your pilotage plan, and practise with the chart reading guide.
When a leading line stops being safe
A leading line is safe only along the stretch shown as a solid line on the chart. Beyond that, the line may cross a bank or rocks, so you must turn off it at the planned point. Note in your plan a clear signal for the turn, such as a buoy coming abeam or a second transit opening. Remember too that leading lines are designed for the draught and size of ships using the port. A yacht may be able to cut corners safely at high water, but at low water the leading line can be the only safe water. Check the tidal height before you leave it.