Module 9 - Compass
Variation
What you will learn
Update charted variation to the voyage date, preserve the east/west sign and explain the precision that the source supports.
Variation is local, dated information. Coastal work should show the epoch, annual change, sign and rounding so another navigator can audit the conversion.
Read the variation statement on the applicable current chart or official source as one unit: value, east/west direction, reference year and annual change. UKHO also publishes magnetic-variation charts with annual rates, but the local chart information remains the practical source for coastal chartwork.
Suppose the rose states 2°30′W (2022), decreasing 8′ annually. For a whole-year 2026 exercise, four changes total 32′. Because the westerly magnitude is decreasing, 2°30′W − 0°32′ = 1°58′W. If the exercise uses whole degrees, state that it rounds to about 2°W rather than silently discarding 58 minutes.
Variation may be east or west and is not constant across the UK. Never assume a nationwide value or direction. The British Geological Survey measures a field that changes with both location and time. A remembered value, a value from a different chart or a vessel's deviation card cannot replace the dated local source.
Worked example
Update 2°30′W in 2022, decreasing 8′ annually, to 2026 using four elapsed whole years.
- 1Annual change: 4 × 8′ = 32′ decrease in the west value.
- 2Updated variation: 2°30′W − 0°32′ = 1°58′W.
- 3For whole-degree working only, record approximately 2°W and state the rounding.
Sense check: A decreasing westerly value must move towards zero. A result of 3°02′W has moved in the wrong direction.
Reject a remembered national correction
A watchkeeper proposes using 2°W because that was used on the previous passage, but the current chart gives an easterly value at a different location and epoch.
1. Use the applicable source
Read the current chart's local variation statement and correction status.
2. Update transparently
Record elapsed time, annual change, E/W direction and any rounding.
3. Sense-check the wording
Confirm that increasing or decreasing moved the stated east/west magnitude in the correct direction.
Sense check: Changing vessel heading affects deviation, not the local variation derived for the same place and time.
Common mistake or limitation
- Treating 'decreasing' as a blind minus sign can reverse a westerly or easterly update. First identify which magnitude the wording modifies.
- A chart rose is dated; using its printed value without applying the stated annual change can preserve a stale correction.
Recap
- Variation belongs to place and date, not the individual vessel.
- Update degrees and minutes from the stated epoch and annual change.
- Keep the sign visible and state the precision of the final value.
Optional quick check
Section 2 of 7
What is 2°30′W in 2022, decreasing 8′ annually, after four whole years?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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