What causes deviation in a magnetic compass?

🔍 Key Concepts

• Compass deviation vs. variation – know the difference between error caused by the ship and error caused by the Earth • Permanent and induced magnetism of the vessel – how the steel hull and equipment affect the compass • Local vs. vessel-specific magnetic influences – which effect moves with the ship and which stays in one geographic area

💭 Think About

• Ask yourself: Which answer describes an error that is UNIQUE to that particular vessel and would change if you switched to a different ship? • Which options describe effects that depend on geographic location or the Earth’s magnetic field, rather than the construction and cargo of the vessel? • Think about what a compass adjuster can correct on a single vessel using magnets and soft iron – which type of error is that?

✅ Before You Answer

• Be clear on the definition: deviation is the compass error caused by the vessel itself, not the Earth • Eliminate any options that describe global or charted magnetic effects (those are not vessel-specific) • Check which option would stay with the ship wherever it sails – that’s the one related to deviation