A compass works by lining its magnetised needle up with the Earth’s magnetic field. Occasionally that needle gets magnetised the wrong way round, so it points south instead of north. That’s reverse polarity, and it’s not something I come across often, but when it happens on a hike and goes unnoticed, it can put you seriously off course. Knowing how it happens, how to spot it, and what to do about it means you’re not relying on a compass that’s quietly lying to you.
Reverse polarity happens when the needle is exposed to a magnetic field strong enough to overwhelm or alter its original magnetisation. In my experience that usually comes down to one of a few sources:
- Strong magnets
- Electronic devices with magnetic components
- Magnetic clasps or closures on packs and cases
- Prolonged contact with speakers, power sources, or electrical equipment
Sometimes it’s a single strong exposure, sometimes it builds up gradually, but either way the needle ends up partially or fully magnetised in the opposite direction.
There are two signs I check for. The most obvious is the needle pointing the wrong way: if the red, north-seeking end consistently points south instead of north, the compass is fully reversed. The less obvious one is unusual needle behaviour, a needle that moves sluggishly, hesitates, or settles inconsistently, which usually means it’s only partially affected. That second one is easy to miss, which is exactly why routine checks matter. If something feels off, compare your compass against known landmarks, your map orientation, or a second compass.
What makes reverse polarity dangerous is that it doesn’t fail in an obvious way. Your bearings can still look perfectly consistent, they’re just consistently wrong, and that’s exactly the kind of error that catches people out in poor visibility, off-track terrain, or when navigating between distant features. Because the error is systematic, it can lead you confidently in the wrong direction, and confidence is the problem: nothing about the compass itself will tell you something is wrong until you check it against something external.
Some sources suggest re-magnetising the needle yourself with a strong magnet. It can work under controlled conditions, but I wouldn’t try it in the field. It’s unreliable outside a proper setting and risks damaging the needle or the capsule further. Once a compass has clearly reversed polarity, I don’t trust it again. Replacement is the safest option.
You could, in theory, treat the south-pointing end of a reversed needle as if it were north and navigate off that instead. In practice I wouldn’t recommend it. It’s error-prone, and it adds cognitive load at exactly the moment you need clarity most. If you suspect reverse polarity, treat the compass as unreliable and stop leaning on it.
Good handling habits go a long way toward avoiding this in the first place:
- Keep your compass away from strong magnets and electronic devices
- Store it separately from phones, GPS units, and magnetic closures
- Avoid placing it near speakers, batteries, or power sources
- Check compass behaviour before every hike
Prevention is far easier than diagnosis out on the trail.
Reverse polarity is rare, but it’s a good reminder that navigation tools can fail in ways that aren’t obvious. I always cross-check compass readings against map features, terrain, and other tools, and if you rely heavily on compass navigation, carrying a second compass is a cheap way to cover yourself. Once you know what reverse polarity looks like, you can recognise when a compass has stopped being trustworthy and adjust your decisions before a small error turns into a serious one.
I managed to fix the Kathmandu compass (shown in the pic) by using the magnet method. Worked like a charm.
This is such a useful Post! Thankyou🤗
Jenny Woodhouse my pleasure.