Module 9 - Compass
Deviation Cards and Swinging the Compass
What you will learn
Explain how a compass swing produces a heading-specific residual-deviation record and use a deviation card without treating it as permission to adjust the compass informally.
A deviation card records the error left after appropriate checking and adjustment. It is evidence for one compass and vessel configuration, not a universal correction table.
A swing compares reliable known directions with the compass indication over a suitable spread of headings. A charted transit can provide a true reference bearing: update and apply variation to establish the corresponding magnetic bearing, then compare that with the compass observation. Other approved methods and equipment may be used by a competent adjuster, but GNSS COG alone is not a reference heading while tide, leeway, manoeuvring or low speed remain material.
The observed pattern helps a competent person assess the compass and make authorised adjustments where required. The final card or curve records residual deviation after adjustment, with the compass, vessel condition, date and headings clearly identified. MCA guidance for regulated ships places adjustment with certificated specialists; this revision explains the navigator's use and monitoring role rather than teaching unqualified adjustment.
At sea, read the value for the intended compass heading. If 030°C is 2°E and 060°C is 4°E, a simple exercise may interpolate 045°C as about 3°E. Real cards and curves can be non-linear, so follow the record's spacing and instructions rather than extrapolating beyond observed headings.
Worked example
A charted transit is 126°T. Updated variation is 2°W and the steering compass reads 121°C while aligned on the transit.
- 1Convert the known reference to magnetic: 126°T − (−2°) = 128°M.
- 2Compare magnetic with compass: deviation = 128°M − 121°C = 7°E for this observed heading.
- 3Record the observation with heading, date, method and conditions; do not infer the complete card from this one point.
Sense check: With east deviation, Compass Least predicts a compass value smaller than magnetic: 121°C is 7° less than 128°M.
| Compass heading | Residual deviation | Navigator's use |
|---|---|---|
| 030°C | 2°E | Use +2° in C→M signed working |
| 045°C | about 3°E | Only if the card supports interpolation |
| 060°C | 4°E | Use +4° in C→M signed working |
Decide whether the old card remains usable
The vessel returns from major electrical work. One transit check differs materially from the old card, but only a single heading has been observed.
1. Limit reliance
Treat the old card as suspect and use independent navigation evidence while the pattern is unknown.
2. Record, do not patch
Keep the observed heading, reference, conditions and discrepancy instead of overwriting one table entry.
3. Arrange competent assessment
A complete swing and any adjustment must follow the applicable vessel requirements and competent procedure.
Sense check: One observation can reveal a problem; it cannot establish residual deviation across the other headings.
Common mistake or limitation
- GNSS course over ground is not automatically a known heading. It is unsuitable as a simple reference when current, leeway, low speed or manoeuvring matter.
- A navigator should not move corrector magnets or rewrite a complete card from one discrepancy; adjustment and documentation follow competent procedure.
Recap
- Known true directions are converted to magnetic before comparison with compass.
- A complete card requires observations over multiple headings and records residual error.
- Use the card within its scope and arrange a fresh check after relevant change or unreliable performance.
Optional quick check
Section 6 of 9
Why can one transit observation not create a complete deviation card?
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.