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Detecting a faulty baseplate compass

A Silva Field 7 Baseplate Compass [1]
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 [2]. 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.

Baseplate compasses, which you might or might not be aware of, tend to be constructed for use within one of five industry-defined zones of the earth within which the magnetic fields act differently. Zone 1 covers most of the northern hemisphere, and the other four zones make up the rest. Australia and New Zealand, and not much else, both fall into zone 5. There’s a clearer explanation, and a map, over here [3].

In different zones, the earth’s magnetic field lines (with which a compass tries to align itself) vary between horizontal and near vertical. If a compass needle could do so, it would completely align itself with the field rather than merely lying flat to the ground. Compass manufacturers typically counter this effect by weighting one end of the needle more than the other, so that the needle remains balanced flat within the compass when the compass is held flat. Some flashy “global” compasses have needles for which the weighting can be adjusted, but if you’re a cheapskate like me then you’d probably buy a compass that’s optimised to work in the zone where you spend most or all of your time. In zone 5’s case the south-pointing end of the needle will usually be weighted so as to couteract the needle’s desire to point its north-pointing end into the ground. Compasses often can be used out of zone, but you’ll need to be careful to ensure you’re holding it on an appropriate angle so the needle is free to move.

It’s not always clearly advertised which way a compass has been weighted. My Silva Field 7 compass has nothing written on it anywhere to identify it as a zone 5 compass, even though I know this model also exists for the northern hemisphere. But I know it’s been constructed for Australia and New Zealand use, because when I hold it flat the needle remains flat. Chances are that if you bought a compass in a northern hemisphere country, that’s where it’s optimised for, and likewise with a compass bought in any other zone unless it’s been parallel imported. Incidentally, this weighting has nothing to do with countering the magnetic offset—in New Zealand it’s still necessary to correct by 23.5° to convert magnetic north to grid north, or something else again for true north.

Back to my original point, the give-away factor which should have told me the needle had flipped was that the needle of my compass was clearly weighted all wrong. I couldn’t hold it flat and have the needle sit nicely. This is because with the heavier southern end of the needle suddenly wanting to point north, and the effect of the weighting was acting in reverse. Rather than countering and neutralising the red end’s inclination to point to the ground, it was now the white end aiming for the ground, and the extra weight at that end was doubling the force!

At the time I didn’t appreciate that this is what was happening until after I’d confirmed my compass was flipped through other means. Once I realised what had happened, the weighting issue made complete sense, but in future I think I’ll immediately make myself very suspicious when I see a mis-weighted baseplate compass, especially if it’s my own one and I’ve used it before without issue.

As a general disclaimer, this insight doesn’t apply to all compasses. It might not apply to all baseplate compasses either depending on the details of how they’ve been manufactured, and I doubt it applies in every zone. Just because the needle isn’t sitting all wonky doesn’t mean it’s magnetically correct, nor does a wonky needle mean that it’s incorrect. Maybe you dropped it, for instance, or maybe you’re using it on a part of the earth for which it wasn’t designed. A wonky needle should always be cause to think that something’s not right, however, and it might be that the needle’s flipped.

It’d be possible to keep using a compass with a flipped needle, as long as one keeps in mind that the red end points to magnetic south instead of north, and I kind of just went with that for the rest of the day, but there were at least a couple of reasons why I didn’t want to do this as a rule.

The first reason was the high potential for confusion—I already have several other similar compasses, and I’d hate to forget which one is the flipped one. It’d also mean that if I loaned it to someone, they could read it completely wrong if they didn’t know the trick. If I have a baseplate compass, I want the red end to point to magnetic north, thank you very much.

The second reason was that I didn’t trust the needle’s new-found state of magnetism at all. It was obviously not magnetised as strongly in the opposite direction as a compass would normally be, as the white end would take a few seconds longer to settle on what it decided was north. If I’m using a compass, I want it to be magnetised properly to the manufacturer’s specifications, and for the same reasons I didn’t bother to consider trying to fix it myself.

I did some research for my own compass once I returned and happily confirmed that Silva has a policy of re-magnetising its compasses for free [4] if the needle flips. Getting this policy fulfilled from outside the USA was a small challenge, because I had to take to back to the retailer who’d never seen such a thing before, and grilled me for a few minutes, I think because he thought the obviously wonky needle was a sign that I’d dropped it, rather than a consequence of the magnetic reversal. Once he phoned and checked with the local distributors, though, it was packed up and sent back to the local distributors for re-magnetisation. Just as well, too, because I don’t think I’d want to keep a compass that’s faulty in such a non-obvious way. At best I’d keep it as a souvenir, and stick a massive label on it warning that it couldn’t be trusted.

2 Comments (Open | Close)

2 Comments To "Detecting a faulty baseplate compass"

#1 Comment By Glen On 5 April, 2012 @ 3:50 am

While the Suunto/Recta “global needle” system does cost more, it does work well, allowing tilt of up to 20 degrees while still giving accurate readings. A useful side benefit is that the needle settles very quickly and without wobble, much like the needle in an expensive dedicated orienteering compass such as the Silva Jet or Suunto Arrow.

#2 Comment By Mike McGavin On 11 April, 2012 @ 12:49 pm

Thanks Glen. Yeah I’m aware that some compasses give more leeway than others. They shouldn’t need to if you’re only using them in the one zone, though, and for the sorts of things I do I’ve found that the simplest usually works best. I found it interesting how the non-balanced needle gave away that there was something very wrong with the magnetism. Cheers.