Module 16 - Pilotage
Transits and Leading Lines
What you will learn
Use an identified transit as a line of position, a track guide or a boundary without treating one line as a complete fix.
Two fixed charted objects in line place the vessel on their extended line. Purpose-built leading marks use that geometry to guide a track, while other transits can confirm a turn or reveal movement across a boundary.
First identify both objects and confirm the intended line on the chart or publication. Similar buildings, seasonal foliage, background lights and an obscured lower mark can create a convincing false pair. A transit observation is valuable because it does not depend on the vessel's compass, but its accuracy still depends on correct identification and visibility.
When the objects align, the vessel is somewhere on that position line—not at one unique point. Combine the line with another independent observation such as a depth contour, cross bearing, distance, abeam feature or electronic position when a two-dimensional fix matters.
A leading line is normally followed by keeping its marks aligned. When they open, the direction and rate of opening show cross-track movement. The pilotage plan should state the correction side and a clearing limit so the helm does not improvise from a confusing night scene.
Use a transit as a turn cue, not an exact fix
The plan says to turn when a church tower aligns with the end of a breakwater. The pair comes into transit, but the echo sounder is two metres shallower than expected.
1. Accept one fact
The vessel is on the transit line if both objects are correctly identified.
2. Reject the unsupported conclusion
The transit alone does not prove the vessel is at the planned intersection or that the turn is safe.
3. Cross-check
Hold off the turn while position, depth, tide and the second planned cue are checked.
Sense check: One line can be precise 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 visually plausible pair is unsafe unless both objects and the intended alignment have been confirmed from the plan.
Recap
- Identify both objects before trusting the alignment.
- Use transits for tracks, boundaries and turn cues with an independent cross-check.
- Monitor how an alignment opens as well as the instant it closes.
Optional quick check
Section 3 of 8
Two planned objects are exactly in transit. What can you conclude from that observation alone?
Continue studying Pilotage
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