According to the Mountaineering Council of Scotland, reversed polarity in compass needles is becoming a significant source of navigation error. The reason is the increasing range of magnetic fields in our transport, clothing and equipment.
Polarity Issues: the Symptoms
With partial polarity reversal the needle becomes sluggish and skittish. With full reversal, the red end will point in the wrong direction!
Compass polarity can be impacted by any strong magnetic field. There are many traps for the unwary:
- Car sound systems
- Mobile phones, tablets and laptops
- Magnets in seat belt buckles
- Avalanche transceivers
- Magnets in clothing fasteners and watch straps
- Car keys...
The reversed needle may not be exactly 180 degrees out so you'll need to fix the needle instead. According to respected manufacturer Silva:
Quickly flick the 'South' pole of a strong magnet outwards along the
'North' end of the needle. Repeat vice-versa. Compare with a compass
that is known to be correct.
Other sources of inaccuracy
Using the compass too close to a ferrous source, such as walking poles, ice axes, cameras, watch, GPS, metal framed glasses, stove, even a wallet (some coins are ferrous). Everyone knows this but it's an easy mistake to make if you are tired or stressed.
Not making proper allowance for magnetic variation.
In some areas such as the Cuillin and Mull in Scotland magnetic rock can deflect the compass needle.
Using a compass in the wrong hemisphere. High end Suuntos have a global balance system. Other compasses are balanced for the hemisphere they were sold in.
Treat your compass as a precision instrument and store it well away from your other electronic devices. Consider purchasing a compass cover that will offer some protection
Don't trust your compass blindly. Develop the habit of checking your needle at the start of every trip and also checking that your bearings are consistent with what you're seeing on the ground while you're on the move.