Module 16 - Pilotage
Transits and Leading Lines
What you will learn
Use an identified transit as a line of position, track guide, turn cue or boundary without treating one alignment as a complete fix.
Two or more fixed marks in one vertical plane define a leading line or transit. The alignment can be precise while still answering only one part of the position problem.
First identify every object and confirm the intended line from the chart or publication. Similar buildings, seasonal foliage, background lights and an obscured lower mark can create a convincing false pair. A transit does not depend on the vessel's compass, but its accuracy still depends on correct identification, visibility and the geometry of the marks.
When the objects align, the vessel is somewhere on their extended position line—not at one unique point. Combine the line with a second bearing, radar range, depth contour, abeam feature, time-distance gate or reliable electronic position when a two-dimensional fix matters.
On a leading line, the direction and rate at which the marks open show cross-track movement. The sensitivity changes with distance and mark separation, so the plan should state the correction sense, intended track and a separate clearing limit. Do not weave around a visual alignment without checking depth, traffic and progress along the channel.
| Use | What alignment proves | What it does not prove |
|---|---|---|
| Leading line | The vessel is on or offset from the charted line | Safe depth and along-track position |
| Turn cue | The vessel has reached the alignment line | The planned intersection without a second cue |
| Boundary | The vessel is approaching or crossing a defined line | Clearance from every other hazard |
Use a transit as a turn cue, not an exact fix
The plan says to turn when a church tower aligns with a breakwater end. The pair comes into transit, but the sounder is two metres shallower than expected.
1. Accept one fact
If both objects are correctly identified, the yacht is on their transit line.
2. Reject the unsupported conclusion
The alignment alone does not prove the yacht is at the planned intersection or that the turn is safe.
3. Cross-check before turning
Protect room while position, depth, tide and the second planned cue are checked.
Sense check: One line can be accurate and still be incomplete; a depth mismatch is evidence to investigate, not noise to ignore.
Common mistake or limitation
- A transit is a position line, not a latitude-and-longitude fix by itself.
- A plausible pair is unsafe unless the marks and intended alignment have been confirmed from the plan.
Recap
- Identify the marks before trusting the alignment.
- Use transits for tracks, boundaries and turn cues with an independent cross-check.
- Monitor how the marks open as well as the instant they align.
Optional quick check
Section 3 of 12
Two planned marks are exactly in transit. What can you conclude from that observation alone?
Sources and factual review
Reviewed 2 September 2026 - Compass Revision editorial review.
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