Module 9 - Compass
Three Norths
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.
- True to magnetic is the variation step for the charted place and date
- Magnetic to compass is the vessel- and heading-specific deviation step
- Suffix every direction T, M or C before doing arithmetic
- Heading, water track and course over ground describe different things
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.