My Silva Field 7 Baseplate Compass.
Usually when intuition says one thing and a compass says something else, it means your intuition is wrong. More than a few times, I or a group I’ve been in have had a compass bearing telling us to go one way that intuitively seemed completely wrong, and after some time it’s turned out that we really were meant to push through an area that looked completely un-navigable. A couple of times, my sense of direction has become bizarrely flipped somehow inside my head, only to be corrected by a compass. On occasion, this has resulted in my sitting down in a flummoxed state for a few minutes trying to flip my head over, but it usually works out.
One problem with a compass, though, is that the needle can occasionally flip—the south pole of the needle becomes north, and north becomes south, meaning the red end of the compass points south instead of north. This is exactly what happened to me on this occasion. The needle flipping actually happened some time before I left, and happened to be wrong when I first pulled it out to use it whilst inside visually encumbering cloud at 1400 metres elevation. Flipped needles are often a consequence of exposure to iron, in the same way that you can easily magnetise a pin by stroking it with a magnet a few times. In my case, I didn’t have a clue what’d caused it because my compass spends most of its time sitting on a shelf, but there are many things that might have caused it. Clearly this can be a problem, and if you’re trying to navigate it can also be a little risky if you’ve not realised what’s happening.
There are a few ways to detect this in the field, though, and I’m keen to hear of any that I don’t mention here. Obviously if you know where you are and can see a known landmark, you can compare the compass to see if the needle’s pointing where you’d expect. If you have multiple compasses, you can compare them and at the very least determine if one compass is misbehaving. In my case, the first sign was that it tried to point me directly back the way I’d just come from, but in most such cases I’d still expect a compass to be more correct than my own intuition. Fortunately my GPS (usually packed away) includes an electronic magnetic compass, which I spent a few minutes calibrating, then compared the two.
There’s yet another sign which might be common knowledge, but I can’t find any references on the ‘net so I thought I might share it. What should have been a dead giveaway for arousing suspicion in my case, especially in hindsight, was that the weighting of the needle within the compass was completely wrong.
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