Module 9 - Compass
Using a Hand-Bearing Compass
What you will learn
Take, time, convert and plot repeatable hand-bearing observations, then judge a fix by geometry and independent evidence rather than the size of its cocked hat alone.
A portable sighting compass can give useful magnetic position lines, but observation quality, magnetic interference, vessel movement, target identity and plotting geometry all affect the result.
Identify a conspicuous charted target before observing it. Stand clear of speakers, phones, steel rigging, current-carrying cables and the steering compass; hold the instrument as designed, allow the card to settle and repeat a doubtful bearing. Record target, magnetic bearing and time.
Apply current variation to obtain the true bearing for plotting, then draw its reciprocal from the charted object towards the vessel. Choose targets that create useful crossing angles. If the vessel moves materially between observations, take the bearings quickly or advance them to a common time when the required accuracy justifies it.
Three lines that do not meet form a cocked hat, but the vessel is not guaranteed to lie inside it. A shared error—wrong variation, magnetic interference, misidentified objects, delayed observations or the same plotting bias—can move all lines together. Compare the result with depth, GNSS, radar range, clearing limits and the safest plausible position.
Challenge a precise but implausible fix
Three neat position lines form a small triangle over shoal water while depth, radar range and GNSS consistently place the yacht farther south.
1. Protect the safe side
Do not average the triangle into a danger area merely because its lines intersect tightly.
2. Audit shared inputs
Check target identity, variation sign, local magnetic interference, observation times and plotted reciprocals.
3. Obtain independent geometry
Use a radar range, depth contour, new target or well-spread bearing to discriminate between the conflicting positions.
Sense check: A small triangle shows precision between the lines; it does not prove accuracy of the common inputs.
Common mistake or limitation
- A portable compass is not immune to deviation. Moving beside a loudspeaker can change the observation immediately.
- The centre of a cocked hat is not automatically the most likely or safest position when errors share a cause.
Recap
- Take identified, timed, repeatable bearings clear of interference.
- Convert and plot reciprocals with useful geometry and a common time.
- Test the result against independent evidence and navigate on the safe side.
Optional quick check
Section 5 of 9
Why can a small cocked hat still be displaced from the vessel's real position?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Compass
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