So, compass errors. They mess with your ability to point at true north, which is kinda important if you're trying to get somewhere. In navigation, you gotta wrap your head around these if you want to steer straight and know where you actually are. The three big ones are variation (some folks call it declination), deviation, and compass error (that's just variation and deviation combined). Oh, and there's heeling error too, which really matters if you're sailing. Each one messes things up differently and you fix 'em different ways. Variation – or magnetic declination if you wanna get fancy – is the angle between true north (that's the actual North Pole, where all the longitude lines meet) and magnetic north (where your compass needle points). It's all because Earth's magnetic field doesn't line up with the planet's spin axis. Where you are on the globe changes it, and it shifts slowly over time 'cause the Earth's core is doing its thing. On a nautical chart, you'll see it printed on the compass rose, and they update it every year. To fix it, you just add or subtract the local variation from your magnetic heading, and boom, you get a true heading. Deviation is the headache caused by magnetic stuff on your own boat. Ferrous metals – you know, iron, steel – plus electrical wiring, electronics, even speakers or batteries can pull your compass needle away from magnetic north. Unlike variation, deviation changes depending on which way you're heading, because the magnetic interference shifts around as you turn. Every boat's deviation is different, and you figure it out by swinging the compass – that's where you turn the ship 360 degrees and note the errors. Then you stick correctors (little magnets or soft iron spheres) near the compass to minimize it. Whatever's left goes on a deviation card. Compass error is just the total gap between what your compass says and true heading. It's variation plus deviation, added together algebraically. Say variation is 10 degrees west and deviation is 2 degrees east – compass error ends up 8 degrees west. Navigators have this dumb mnemonic "True Virgins Make Dull Companions" (or "Cadbury Dairy Milk Very Tasty" if you prefer chocolate) to remember the order: True heading + Variation = Magnetic heading + Deviation = Compass heading. To get a true bearing from a compass bearing, you apply the error the opposite way. You check compass error regularly by comparing it to a known bearing, like a transit line or a star sight. Heeling error is a dynamic bugger that pops up when your vessel leans to port or starboard – usually under sail or in rough weather. It happens because the vertical part of Earth's magnetic field messes with the compass's internal magnets. When the boat heels, the compass card tilts, and that vertical field creates a horizontal force that deflects the needle. It's worst on headings near north or south, and can be a real pain on yachts with deep keels. Compasses for sailboats often have special correctors (small magnets or a gimbal system) to tame it. You can't fully get rid of heeling error, but good compass selection and installation help a lot. You gotta check your compass regularly to keep it trustworthy. Here's a quick list for verifying those errors: Nah, but you can get 'em down to tiny values – usually less than 1-2 degrees – with proper correction. Variation is just nature, you can't eliminate it, only account for it. Deviation you can minimize with careful corrector placement, but there's always some residual. Heeling error is the toughest to fix completely; good compass design and installation are your best bet. At least once a year, or after any major change to the boat – like installing new electronics, swapping metal fittings, or after a lightning strike. If your compass reading is off by more than 3-5 degrees from a known bearing, it's time to swing it again. True north is the geographic North Pole, where all longitude lines come together. Magnetic north is a point in northern Canada where Earth's magnetic field points straight down. The angle between 'em is variation (or declination). Charts and GPS use true north; your compass needle points to magnetic north. Hell no. GPS gives you true heading and position, but it can fail – signal loss, jamming, power failure. A magnetic compass is your critical backup. Knowing compass errors means you can navigate accurately even without electronics. Plus, lots of folks still use magnetic compasses for steering 'cause they're simple, reliable, and don't need power.What are the three types of compass errors
What is variation (declination) in compass navigation?
What is deviation in a magnetic compass?
What is compass error and how is it calculated?
What is heeling error and when does it occur?
Compass errors comparison table
Error type
Cause
Does it change with heading?
How to correct
Variation
Earth's magnetic field (geographic vs. magnetic poles)
No (changes with location and time)
Apply local variation from chart; update annually
Deviation
Local magnetic fields on the vessel (metal, electronics)
Yes (varies with ship's heading)
Swing the compass; use correctors; record on deviation card
Heeling error
Vertical magnetic component when vessel heels
Yes (appears during heel, especially on N/S headings)
Heeling error correctors; gimbal mounting; reduce heel
How to check compass errors on your vessel
Frequently asked questions about compass errors
Can compass errors be completely eliminated?
How often should I swing my compass?
What is the difference between magnetic and true north?
Does GPS eliminate the need to understand compass errors?
Resumen breve
Related articles
- What are the three types of compasses
- What are the three types of navigation
- What are the three types of volunteering
- What are the three main types of navigation
- What are the three types of lasers
- What are the three types of sails
- What are three types of lasers
- What are the three types of wind indicators