Compass Variation: What Are Variation and Deviation?
Two separate differences between the compass north and chart north — one caused by the Earth, the other by your own boat.
A compass shows north — but which north? A chart is drawn to true (geographical) north, while a magnetic compass points to the Earth’s magnetic field. Two corrections deal with the difference, and the two are often confused. The table below makes it clear.
Two corrections
Term | What is it? | Source |
|---|---|---|
Variation (declination) | The difference between true north and magnetic north | Depends on the area and the year; shown on the chart |
Deviation | The boat’s own metal and electronics throw the compass off | Unique to each boat, and even to each heading |
Variation is a matter of geography and is given on the chart (it changes slowly over time). Deviation is your boat’s personal signature: speakers, engine, phone, tool bag, and any bit of iron near the compass will upset it, and that error can change depending on which way the boat is pointing.
In the GPS age people ask whether it is still needed, but there are two solid reasons: if GPS fails or the power goes off, the magnetic compass is your only direction source, and at that point you need to know the correction. Practical advice is simple: don’t leave metal or a phone near the compass (it will upset the deviation immediately), and keep a deviation card aboard. A memory phrase helps keep the order straight: 'True Virgins Make Dull Companions' — it reminds you of the sequence when converting to compass readings.