What is magnetic compass deviation?

Master the Navigation, Seamanship, and Shiphandling (NSS) – Officer of the Deck (OOD) Phase 2 Exam with engaging flashcards and multiple choice questions, complete with hints and explanations. Prepare comprehensively for your test!

Magnetic compass deviation is the error in a compass reading caused by the ship’s own magnetic influence. The vessel’s hull, ferrous fittings, machinery, and electrical equipment create local magnetic fields that distort the Earth’s magnetic field inside the ship, so the compass does not point exactly at magnetic north. This deviation changes with the ship’s heading and with changes to the ship’s configuration, and it is mapped in a deviation card after swinging the compass. It’s separate from variation, which is the difference between true north and magnetic north due to Earth’s magnetic field. Wind-induced effects or water salinity don’t cause this kind of deviation.