Transits: the most accurate position line
1The lesson in plain words
When two charted objects line up, you are on the line through them. No compass is involved, so a transit is the most accurate position line you can get.
What a transit is
A transit is two charted objects seen exactly in line, one behind the other: a beacon and a headland, two edges of land, a pair of leading marks. You are on the straight line drawn through them and extended towards you. Spanish charts and courses call it an enfilación.
Why it is so good
A transit needs no compass, so there is no compass error, variation or deviation to apply. It is also sensitive: the further apart the two objects are, compared with your distance from the nearer one, the sooner you see them open when you move off the line. That makes transits useful for fixing your position and checking the compass.
Fixing with a transit
Cross the transit with a compass bearing taken at the moment the objects come in line, choosing an object whose bearing line crosses the transit near 90°. Plot the transit as a line through both objects, then the bearing from its object: where they cross is your fix. Pick useful transits from the chart before you need them.
Checking the compass
Measure the true bearing towards the aligned marks on the chart; convert it to magnetic. Sight them with the compass’s bearing attachment and record the boat’s heading. Compare magnetic minus compass for deviation, the boat’s compass error on that heading. True minus compass gives total compass error, or corrección total. Reduce either difference to within ±180°: positive is east, negative west. Following the line over the ground does not prove your bow points along it.
Natural transits
Edges of land, a headland and a distant island, or a beacon and a hilltop all work, as long as both are charted and you are sure what they are. A transit using a buoy is less reliable, because the buoy can shift.
2Key data
- Compass error
- None: a transit needs no compass
- Uses
- Position fixing and compass checks
- Sensitive transit
- Objects far apart; the nearer one close to you
- Spanish terms
- Enfilación; corrección total from an enfilación
3Key points
- 1
A transit is two charted objects in line: you are on the line through them.
- 2
It needs no compass, so it is the most accurate position line.
- 3
Cross the transit near 90° with a bearing taken as the objects line up.
- 4
Compare the transit’s magnetic bearing with its compass bearing; deviation applies to the boat’s heading during the check.
4Frequent mistakes
Wrong: Apply variation and deviation to a transit before plotting it.
Right: Draw it straight through the two charted objects. No compass is involved.
Wrong: Any two objects in line make a transit.
Right: Both must be charted and positively identified. A buoy can be out of position.
5Practice case & exam keys
Your bow points towards two charted beacons in line along 180°T. Your steering compass reads 186°C, and variation is 2°W.
Show the answer
The transit’s magnetic direction: 180°T with 2°W variation is 182°M. The compass reads 186°C, 4° more than magnetic, so deviation on this heading is 4°W. Total compass error is 6°W: 2°W variation plus 4°W deviation. This result does not establish deviation on other headings.
Exam keys
- Transit: two charted objects in line; no compass error.
- The most accurate single position line.
- Fix: a transit plus a simultaneous bearing, crossing near 90°.
- Deviation = magnetic − compass; total error = true − compass, reduced to ±180°. East positive, west negative.
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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