Module 9 - Compass
Three Norths
What you will learn
Distinguish true, magnetic and compass north and identify which error belongs between each pair.
A charted direction, the Earth's magnetic field and the vessel's steering compass can use three slightly different north references. Safe conversion starts by naming the reference on every course or bearing.
True north is the geographic chart reference. Magnetic north is the direction of the Earth's magnetic field at the place and date. The angular difference between true and magnetic is variation, derived from current chart information rather than assumed for a whole country.
Compass north is the north indicated by a particular installed compass in its current magnetic environment. The vessel's engine, wiring, speakers, steel and equipment can displace it from magnetic north; that heading-dependent difference is deviation.
Write the suffix every time: 084°T, 087°M or 085°C. An unlabelled number hides which corrections have already been applied and makes a plausible-looking error difficult to detect. GPS course over ground is a movement direction and is not a fourth compass north or a substitute for heading.
Variation separates true from magnetic; deviation separates magnetic from this compass.
Label a mixed-reference pilotage note
A charted clearing line is 084°T, the hand-bearing compass reads 087° and the steering compass heading is 085°C.
1. Name each source
The chart line is true, the clear hand-bearing reading is magnetic, and the installed steering compass is compass.
2. Do not compare raw numbers
Convert observations to one reference before deciding whether a limit has been crossed.
3. Keep movement separate
Use track or course over ground as an independent drift check, not as proof that the compass conversions are correct.
Sense check: If the suffix disappears, it is impossible to tell whether 3° variation or deviation has already been included.
Common mistake or limitation
- GPS course over ground is not true heading: tide, leeway and manoeuvring can make them differ.
- Variation and deviation are separate errors from separate sources and must not be merged before the steps are understood.
Recap
- Charts use true; an undisturbed magnetic compass senses magnetic north.
- An installed compass adds the vessel's heading-specific deviation.
- Label every value before comparing or converting it.
Optional quick check
Section 1 of 5
Which error belongs between magnetic north and the installed steering compass's north?
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.