Module 9 - Compass
Deviation
What you will learn
Use heading-specific residual deviation and decide when the installed compass, its card or its magnetic environment requires investigation.
Deviation is not one permanent boat correction. It belongs to a particular compass, vessel configuration and heading, and it can change when equipment or magnetism changes.
Permanent and induced vessel magnetism, engines, steelwork, wiring, speakers, current-carrying equipment and portable items can alter a magnetic-compass reading. As the vessel turns relative to the Earth's field, both the size and sign of the residual deviation may change around the card.
A compass swing observes reliable known directions over a range of headings. After authorised adjustment where applicable, the residual errors are recorded as a table or curve. Use the entry for the compass heading concerned and interpolate only when the card and operating procedure support it.
The MCA advises checks after nearby electrical or magnetic equipment is added, removed or altered, after relevant structural work, when the compass becomes unreliable and when records indicate excessive error. Portable electrical or steel items should be kept away. A surprising difference is a reason to cross-check and investigate, not to invent a correction from one observation.
Respond to a changed magnetic environment
After a cockpit loudspeaker is replaced, the steering compass differs by 4° on a known transit while a clear hand-bearing observation and the charted direction agree.
1. Remove immediate interference
Move portable items and inspect whether the new speaker or wiring is inside the compass's safe separation area.
2. Use independent navigation
Do not rely on the old card while the source and heading pattern of the difference remain unknown.
3. Arrange a proper check
Have the compass and residual-deviation record checked by a competent person before treating the previous corrections as valid.
Sense check: Variation cannot explain a new difference that appeared immediately after equipment changed aboard at essentially the same place and time.
Common mistake or limitation
- Deviation is not one fixed value to add on every heading; the sign and magnitude can change around the compass card.
- A hand-bearing compass is also vulnerable to local magnetic interference and does not automatically validate the steering compass.
Recap
- Read residual deviation for the relevant compass heading.
- Treat the card as configuration-specific evidence, not a permanent property of the vessel.
- Cross-check, restrict reliance and investigate when equipment or observed performance changes.
Optional quick check
Section 3 of 7
Why can the same residual-deviation value not be assumed on every heading?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Compass
This public lesson is part of Revision Module 9. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.