Transits, leading lines and sector lights
1The lesson in plain words
Two fixed marks in line give you a line that needs no compass. Learn to follow one, and how to get back onto it.
Transits
A transit is when two fixed objects appear one behind the other. You are then somewhere on the straight line through them, extended towards you. A transit needs no compass, so it has no compass error. Sensitivity depends on the marks: a nearby front mark and a more distant rear mark show sideways movement more clearly than two distant marks close together. Use charted, fixed objects; a buoy swings round its mooring.
Leading lines
A leading line is a transit chosen to lead you safely into or along a channel. It is often marked by two leading marks or leading lights, with the rear one higher than the front. The chart shows the line and its bearing. Stay on it until the turning point in your plan, then leave it as planned.
Getting back on
When approaching marks ahead, if they separate, steer towards the front mark, the nearer one: it has moved the way you need to go. Example: the front mark appears to the right of the rear mark, so steer to starboard until they line up again. In a cross-stream, point the bow into the stream enough to keep the marks aligned.
Looking astern
On departure, a back bearing is a compass bearing to a mark astern; a stern transit uses two fixed marks astern. With the bow pointing along the outward leg, look directly aft: nearer mark left of farther means correct to starboard; nearer mark right means correct to port. Allow for stream and keep checking alignment. Practise in safe water. For example, departing on 225°M, keep the mark astern bearing 045°M; compass error still applies.
Sector lights
At night many entrances use a sector light: one light that shows a different colour depending on the direction you see it from. A narrow white sector usually marks the safe line, with red on one side and green on the other. The chart shows which colour is on which side; there is no single rule. Light characters are in the IALA module.
2Key data
- Transit
- Two fixed marks in line; no compass error
- Off the line
- When approaching marks ahead, steer towards the nearer mark
- Leading marks or lights
- Rear mark higher than the front; bearing on the chart
- Sector light
- Colour depends on the direction you see it from
- Sector colours
- Check the chart for which colour lies on which side
3Key points
- 1
A transit avoids compass error. Choose fixed, charted marks with good separation in distance.
- 2
When approaching marks ahead, steer towards the front mark if they separate.
- 3
In a cross-stream, point the bow into the stream enough to keep the marks aligned.
- 4
In a sector light, the white sector usually marks the safe line; check the chart.
4Frequent mistakes
Wrong: With marks ahead, steer towards the rear mark when they separate.
Right: When approaching marks ahead, steer towards the front, nearer mark. It shows the side the line is on.
Wrong: A buoy in line with a hilltop makes a good transit.
Right: A buoy moves round its mooring. Use two fixed, charted objects.
Wrong: In a sector light, red is always on the port side.
Right: There is no single rule. Read the sector colours and bearings on the chart.
5Practice case & exam keys
You are entering on a leading line of 045°M. The front, lower mark appears to the left of the rear mark, and a cross-stream is setting you to the east. What do you do?
Show the answer
The front mark has moved left, so you are to the right of the line: steer to port, towards the front mark, until the marks line up. The stream sets you east, to starboard of your heading, so once back on the line hold the bow to port of 045°M as needed to keep the marks aligned. Keep watching the marks; adjust as soon as they start to open.
Exam keys
- Transit: two fixed marks in line, no compass error.
- When approaching marks ahead, steer towards the front mark if they separate.
- Leading line: the rear mark stands higher; the bearing is on the chart.
- Sector lights: colour changes with direction; read the chart.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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