Module 9 - Compass
Variation
What you will learn
Update a charted variation to the voyage date and apply its east or west sign without assuming a national value.
Variation changes with place and time. The chart gives a reference value, epoch and annual change; the navigator updates that information and checks the chart's correction status.
Read the compass rose or other current official chart information carefully. A statement such as 2°30′W (2022), decreasing 8′ annually, means the west value reduces by eight minutes each year after 2022. Convert minutes consistently and apply the elapsed whole or proportionate years required by the exercise or planning method.
For August 2026, four years after 2022 in a whole-year classroom calculation, 4 × 8′ = 32′. Subtracting that from 2°30′W gives 1°58′W, commonly rounded to 2°W if the chartwork convention requires whole degrees. State the rounding rather than silently discarding minutes.
East/west is part of the value. For signed working in this course, east is positive and west negative when moving Compass → Magnetic → True. Variation may be east or west, so never assume a nationwide value or direction: derive it for the charted place and date.
Worked example
Update 2°30′W in 2022, decreasing 8′ annually, to 2026 using four elapsed years.
- 1Annual change: 4 years × 8′ = 32′ decrease in the west value.
- 2Updated value: 2°30′W − 0°32′ = 1°58′W.
- 3If whole-degree chartwork is required, round and state the result as approximately 2°W.
Sense check: A decreasing westerly value moves towards zero; adding 32′ to make 3°02′W has applied the wording backwards.
Reject a stale remembered correction
A crew member says 'variation is always west here' while the chart rose gives an east value for a different location and later epoch.
1. Use the local source
Take the reference value and annual change from current applicable chart information.
2. Update transparently
Show elapsed time, arc-minute change, sign and rounding.
3. Cross-check the result
Ask whether an increasing or decreasing east/west value moved in the stated direction.
Sense check: A nationwide mnemonic cannot override a dated local chart value.
Common mistake or limitation
- 'Decreasing' modifies the magnitude named on the rose; it does not always mean subtract a signed number without thought.
- Variation is not taken from the vessel's deviation card and does not change merely because the boat changes heading.
Recap
- Variation belongs to the place and date, not the individual boat.
- Update degrees and minutes from the stated epoch and annual change.
- Keep the E/W sign and state the chosen rounding.
Optional quick check
Section 2 of 5
What is 2°30′W in 2022, decreasing 8′ annually, after four years?
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.