Checking gyro error: transits, celestial bearings and comparisons
1The lesson in plain words
Compare a known true bearing with the gyro bearing of the same thing. The difference is the gyro error. Check it every watch where you can.
The rule
STCW Code A-VIII/2 requires you to determine standard magnetic compass error at least once a watch and, when possible, after a major course alteration. Compare standard and gyro compasses frequently; keep repeaters synchronised. Check gyro error regularly under your ship’s procedures. Gyro error = true − gyro. Use the signed difference within ±180°: 001° true minus 359° gyro is +2°, or 2° E. Away from the 000° crossing, high readings mean W error; low readings mean E error.
By transit
A transit is two charted objects seen in line. Its true bearing is measured from the chart, or listed for a leading line. As the objects come in line, take the gyro bearing. This is a useful coastal check: it gives a charted true bearing without a separate position fix. Choose objects that are well separated and clearly identified, and take the bearing at the moment they line up.
Worked: by transit
From the chart, a transit of two beacons bears 052.0° T. As they come in line, the port bearing repeater reads 053.5°. Gyro error = 052.0 − 053.5 = −1.5°, that is 1.5° W: the gyro reads 1.5° high. To correct gyro readings to true, subtract 1.5°. A gyro heading of 230.0° is 228.5° T.
Other ways
At sea, use a celestial body. Its true azimuth or amplitude is calculated for the time and position; the Compass error by azimuth and amplitude unit does the working. From a known fixed position, you can compare the gyro bearing of a distant charted object with its true bearing from the chart. For a magnetic comparison, first apply known variation and deviation to obtain true heading; an independently verified GNSS compass provides another reference.
Comparing with magnetic
The same moment gives the magnetic compass error. In the example, the standard compass reads 236.0° on the heading that is 228.5° T. Magnetic compass error = 228.5 − 236.0 = −7.5°, or 7.5° W. The chart gives variation 3.0° W, so deviation = 7.5° W − 3.0° W = 4.5° W on this heading. Log it against the deviation card.
2Key data
- Gyro error
- True − gyro, wrapped to ±180°; positive = east
- Gyro reads high (away from 000°)
- Error is westerly: subtract it to get true
- Gyro reads low (away from 000°)
- Error is easterly: add it to get true
- Standard compass error (STCW)
- At least once a watch; after major turns where possible
- Deviation
- Magnetic compass error − variation
3Key points
- 1
Gyro error = true − gyro, wrapped to ±180°: positive = E (add); negative = W (subtract).
- 2
A transit gives a reliable charted true bearing without a separate position fix: a useful coastal gyro check.
- 3
Offshore, use an azimuth or amplitude, worked in the Compass error by azimuth and amplitude unit.
- 4
Use a checked gyro error and current variation to derive magnetic deviation from simultaneous compass headings.
4Frequent mistakes
Wrong: Gyro error is gyro bearing minus true bearing.
Right: Gyro error = true − gyro. With 052.0° T and 053.5° by gyro, the error is −1.5°, or 1.5° W.
Wrong: A westerly gyro error is added to gyro readings.
Right: A westerly error means the gyro reads high, so you subtract it. Add an easterly error.
Wrong: One check at departure is enough for the voyage.
Right: STCW specifies standard-compass error at least once a watch, and after major turns where possible. Check gyro error regularly under ship procedures, compare compasses frequently and log results.
5Practice case & exam keys
Entering port, a leading line bears 317.0° T on the chart and 315.5° by gyro. Your gyro heading is 317.5°. A colleague proposes subtracting 1.5° from gyro readings. Should you accept that, and what is the true heading?
Show the answer
No. The error is 1.5° E, so add it. Gyro error = 317.0 − 315.5 = +1.5°. The true heading is 317.5 + 1.5 = 319.0° T. Log the check with time, method and both bearings, and tell the master if the error differs from the last one.
Exam keys
- STCW Code A-VIII/2: determine standard magnetic compass error at least once a watch and after major course alterations where possible; compare compasses frequently and keep repeaters synchronised.
- Gyro error = true − gyro, wrapped to ±180°: positive → E, add; negative → W, subtract.
- Methods: transit or leading line, celestial azimuth or amplitude, true bearing from a fixed position, comparison with magnetic.
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.
My notes for this unit
No notes