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Compass basics

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 Kenny 31 Jan 2005
Hey ho it's time for me to buy a new compass as I think I might start a bit of hillwalking again this year. USed to loads (well, more moors-walking) years ago in the Scouts, had a Silva Type 4 which eventually died (most of the damping fluid disappeared! Did it diffuse, or evaporate, or both? Have binned it anyway, it had a good innings).
Looked at a few threads where the very good point is raised that the Type 4 is somewhat oversized for any realistic requirements.
Suunto M3 seems to be recommended but now I see there's a Suunto M3 Global "with a needle that can be used anywhere in the world". Being naive in such matters, I don't understand this, could someone enlighten me? i.e. if one day I decided to go trekking in Namibia, would a non-global compass be no use?
Also curious to know whether the "curvimeter" on the Suunto A40 is more than just a gimmick for the lazy route-planner?
One here:
http://www.needlesports.com/cgi-bin/sh000003.pl?REFPAGE=http%3a%2f%2fwww%2e...

So basically I am after general recommendations for a reasonably priced practical compass for general walking, and what this "global" issue is.
Cheers
K
 Ridge 31 Jan 2005
In reply to Kenny:
Compass needles, as I recall, require different degrees of damping where you are in the world. Not sure how the global needle system works, but I'm sure someone else will.
Personally I like the Type 4, luminous bits for night nav, pretty much everything you need. I don't mind the large base, makes triangulation easier, IMHO.
 GrahamD 31 Jan 2005
In reply to Kenny:

The downward component of the earth's magnetic field varies from place to place. Global compasses are supposed to be able to compensate for this (don't know how) whereas bog standard compasses are balanced only for UK/Europe.
 Carolyn 31 Jan 2005
In reply to GrahamD:

Does this mean (e.g.) a 'european' compass isn't going to work in Africa, or does it just mean it's going to take longer for the needle to settle?
 Ridge 31 Jan 2005
In reply to Carolyn:
From the tracklogs website:

Q. Does my compass work anywhere in the world?


Global Voyager Range
Yes, the only thing you might consider is the fact that the compass needle can dip or tilt if you travel far north or far south from the area in which you bought the compass (and for the area in which the compass was intended to be used).

Normally this does not affect the accuracy of the compass but for world wide travellers we recommend the Voyager series. The tilting effect is handled by the construction and design of the needle balancing used in this compass range.

Magnetic declination will also vary as you move throughout the world. If you frequently move between areas with differing magnetic declination it will significantly simplify your compass usage if you select a unit with declination compensation. Compass models featuring declination compensation are the Ranger 15 TDCL, the Voyager range and the Eclipse range.


 Ridge 31 Jan 2005
In reply to Ridge:
Having no knowledge of magnetism, I'm guessing that the further south you go the more the needle tries to point 'down', towards the magnetic pole?
 Carolyn 31 Jan 2005
In reply to Ridge:

My very rusty memory of magnetism suggests it depends which end of the needle you're talking about! But that's largely irrelevant, I think - around the equator the needle's going to lie relatively flat, as you move further north it's going to tip in one direction, and as you move south, it'll tip in the opposite direction. This 'tipping' of the needle can make it 'stick', or take longer to settle.

I wait to be corrected.....
 GrahamD 31 Jan 2005
In reply to Carolyn:

Thats pretty much how I understand it but its outside my experience, I'm afraid.
KarenR 31 Jan 2005
In reply to Carolyn: You are basiclly correct. In the northern hemisphere the magnetic field points north and down into the earth. In the southern hemisphere it points north and up out of the earth. The angle the field makes with the earth gets bigger as you approach the poles, so that if you are standing on the magnetic north pole your compass will try and point vertically downwards.

When a compass is made the needle is weighted so that it will lie horizontally at a given latitude. So, if you take your compass made in the UK too far north or south you will struggle. It should be useable down to the equator I guess. I took a UK compass to New Zealand, and it was OK as guide, but not good for any serious navigation. The problem isn't that the needle takes longer to settle, but that it points so steeply upwards that it gets stuck against the cover.

Compasses that are made to be used all over the world either allow you to take them apart to adjust the weighting on the needle, or mount the needle in such a way that it can pivot vertically as well as horizontally.

So I guess it depends on where you are likely to be using your compass as to what you want to buy.
OP Kenny 31 Jan 2005
In reply to Kenny:
Thanks all, very useful stuff. If the Suunto example is anything to go by, a "global" compass is about 50% extra in cost than a "Europe" one. So I'll have to work out how far north or south I'm likely to go. Might just get the really basic Silva one for a tenner and shell out for a posh global one when I actually need it, then I'll still have a small light spare one anyway.
Quinag 31 Jan 2005
In reply to Kenny:
Quote:
The vertical intensity and direction of earth's magnetic field, the inclination, influences the horizontal plane of a compass needle according to the latitude where it is used. Due to the inclination the compasses must be balanced for several zones in order to keep the needle in horizontal position.

So if you're planning to go on expeditions in different latitudes, get the global model.

If you are looking for "extra" features that are useful in the field, I would go for declination adjustment (saves mental arithmetic and fiddly adjustments when working with a map), a mirror (gives you far more accurate sight bearings, which is helpful when you're completely lost) and a magnifying glass if your arms are getting too short like mine are these last few years.

 Conan 31 Jan 2005
In reply to Kenny:
All you need to do is tilt the compass slightly to compensate for the dip. Simple solution that worked for me in New Zealand a couple of weeks ago.
 Carolyn 31 Jan 2005
In reply to Quinag:
> If you are looking for "extra" features that are useful in the field, I would go for declination adjustment (saves mental arithmetic and fiddly adjustments when working with a map)

But make mighty sure you set your magnetic variation in the right direction to start off with (there was a bloke on my winter ML training who finally worked out he'd set it wrong after 3 days of getting lost in whiteout conditions....)
OP Kenny 02 Feb 2005
In reply to Kenny:

Right. The Silva Voyager 8010 looks good, I can get it for £20 in a shop down the road. But I am curious about the reasoning about making bright yellow baseplates - anyone? Might take me some time to get used to the relatively large and black ring too. Anyone care to furnish some horror stories about this one?!
Anonymous 02 Feb 2005
In reply to Quinag:

Declination adjustment options just confuse me, I'm so used to always amending the readings that I tend to change them even if there's a pre-set adjustment on the compass.

I have never, ever needed to take a sight bearing. If you can see far enough to find something to take a bearing to, then you can see far enough to work out where you are, surely? I've always thought mirrors were a silly idea on compasses, and they make them bigger and heavier too.

Magnifying glasses are a good idea though...

Victim of Mathematics
OP Kenny 02 Feb 2005
In reply to Kenny:

oh yes thanks Quinag but I decided against a mirror-equipped for now. never used one when I used to do lots of navigation all those years ago and I'm not looking to do anything particularly precise or advanced at the moment, just basic hillwalking of the kind that doesn't actually really need a compass (but of course one must have one on oneself!). as for declination adjustments...well it seems that most of the modern ones have this; I seem to remember just adding or subtracting six as appropriate (this was around 1991, obviously changed now)

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