A magnetic compass indicates direction using Earth’s magnetic field, and remains mandatory backup equipment on every vessel despite modern gyrocompass and GPS systems.
Why It’s Still Required
Gyrocompasses and GPS depend on electrical power and electronic systems that can fail. A magnetic compass works independently of any electrical system, making it the genuine last-resort navigation reference when everything else fails — which is exactly why it remains mandatory rather than a legacy holdover.
Correcting Magnetic Compass Readings
A magnetic compass reading isn’t the true direction directly — it requires two corrections:
- Variation — the difference between magnetic north and true north at the vessel’s current location, shown on charts
- Deviation — additional error caused by the ship’s own magnetism, varying by heading, tracked on a deviation card specific to that vessel
Deviation Card Maintenance
A vessel’s deviation card must be periodically checked and updated, since the ship’s magnetic properties can change over time (structural work, cargo changes, equipment changes) — an outdated deviation card undermines the entire point of carrying a magnetic compass as a reliable backup.
Exam Relevance
Magnetic compass principles, variation/deviation correction, and deviation card maintenance are foundational Navigation topics in deck officer competency exams.