I was assisting on a navigation training exercise a few years ago when a pair of the trainees appeared at a path junction and proceeded to work out which track they should take next.

Both trainees quickly and efficiently oriented their maps to north using their compasses and declared “That way” at the same time.

DSC_0321Unfortunately they were pointing in opposite directions!

There was no problem with the skills used to orient the map but one of the compasses was pointing south instead of north, so what caused the error, and what else can you do to break such an essential piece of kit?



This is where a compass no longer points in the correct direction. Whilst many people speak about compasses switching to point south instead of north, the error may not be so simple. Your compass could switch by any number of degrees from 0 to 359.

Depolarisation is caused by subjecting the compass to a strong magnetic field. Unfortunately for navigators, 21st century technology means that it’s hard to avoid strong magnetic fields!

A few examples of things I’ve known wreck a compass are:
• Mobile phones (the speaker has a magnet inside)
• Home electronics (electromagnetic fields surround the cables in our homes)
• GPS devices
• Magnets on rucksack chest straps, designed to keep hydration bladder mouthpieces in place.

Your compass can also be temporarily thrown off course by using it too close to some metal objects (such as cars made of steel with an iron engine block) or electromagnetic fields generated by electricity cables.

The needle in most compasses sits inside a DSC_0093sealed capsule of fluid (often white spirit, paraffin or another mineral oil). Small bubbles can appear inside the capsule at times, caused by changes in temperature or air pressure. This is not a major concern, but larger bubbles that don’t disappear when the compass is gently warmed could be a sign that the liquid is leaking out, often caused by a lack of care when handling the compass, for example dropping it. This can cause inaccuracies and a poorly responsive needle.

Travel to Skye!
The Isle of Skye is a fantastic walkers’ paradise, attracting thousands of visitors every year attempting the Cuillin Ridge. Alongside being a fantastic mountaineering challenge though, the Cuillin offers a first rate example of the deviousness of the ground beneath your feet!
The rough Gabbro rock that forms the Cuillin has magnetic properties and has been known to send compass needles into an endless, lazy spin! Although Skye is not the only place where this phenomenon can be seen, it’s among the best known.

How to repair your compass?

There are a few different ways to repolarise a compass by using another magnet but, if yours is a Silva compass, probably the easiest way is to send it back to the manufacturer! Although they offer only a 2 year warranty they’ve been known to repair or replace compasses many years older than that.
Better still would be to avoid breaking the compass in the first place! A few simple steps should ensure your compass never sends you off course, so here goes…
Store your compass away from any strong magnets. And electrical cables, devices or appliances. And mobile phones. And speakers. Don’t spend too long hanging around electricity pylons and telephone wires when you’re using your compass.

In fact, it’s probably best if you store the thing in a thick wooden box, buried in the garden, at least 10 metres from the telephone line into the house (as long as your house isn’t built on gabbro!)

Joking aside, a compass is an incredibly useful part of your navigational toolkit, but it’s not foolproof. The best defence against a faulty compass is to check it’s accuracy before each use and to develop good map reading skills so that you’re never entirely reliant on that little red magnet!

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