Module 9 - Compass
Three Norths
What you will learn
Distinguish true, magnetic and compass references, then keep heading, water track and course over ground separate in a Yachtmaster command decision.
Advanced compass work fails when a correct number is attached to the wrong reference. Label every course and bearing before conversion, and treat movement over the ground as separate evidence rather than another kind of north.
True north is the geographic direction used for the meridians and bearings on the nautical chart. Magnetic north is the horizontal direction of the local Earth magnetic field. A compass needle aligns with that local field; it does not point along a simple great-circle line to one magnetic pole. The true-to-magnetic difference is variation and changes with place and time.
Compass north is the direction indicated by a particular installed compass in its present vessel environment. The magnetic-to-compass difference is deviation, which can change with heading and after electrical, magnetic or structural changes aboard.
A vessel can be heading 084°T while its water track and GNSS course over ground differ because of leeway, tidal stream, manoeuvring or sensor lag. Suffix directions T, M or C and name heading, bearing or track. An unlabelled 084° hides which corrections and physical quantity are already included.
Variation separates true from magnetic; deviation separates magnetic from this compass.
Reconcile three plausible directions
The pilotage plan gives a clearing bearing of 084°T, a hand-bearing observation reads 087°M, the steering compass shows 085°C, and GNSS reports 091° course over ground.
1. Name the quantities
The clearing line is a true bearing, the clear portable observation is magnetic, the installed compass is compass heading, and COG is movement over the ground.
2. Convert before comparing
Use current variation and heading-specific deviation so the bearing and heading share one reference.
3. Explain the residual
Only after conversion should tide, leeway, steering variation or observation error be used to explain the remaining difference from COG.
Sense check: If a raw compass number is compared directly with COG, two different references and two different physical quantities have been mixed.
Common mistake or limitation
- Magnetic north is the direction of the local horizontal field, not a needle pointing geometrically at one magnetic pole.
- GNSS COG is not true heading. It can agree in specially controlled conditions, but tide, leeway and manoeuvring normally make that an assumption to prove.
Recap
- Variation separates true and magnetic; deviation separates magnetic and this compass.
- Suffix every direction and identify whether it is a heading, bearing or track.
- Convert references first, then use independent movement evidence to diagnose the difference.
Optional quick check
Section 1 of 9
Which item is not one of the three north references used in compass conversion?
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.