UKC

Hi,

I have two queries about compasses:

1 - Do compasses work differently in different hemispheres? I recently asked this in Tiso's thinking it would possibly be a silly question but was told that, that was in deed the case and not a silly question. So, when in New Zealand after this, I bought a compass there for our hiking rather than bringing my old one from the UK. However in the NZ outdoor shop they didn't think this was the case but having left the other I bought one anyway.

2 - If you have two compasses and they do not point in the same direction, even when not close enough together to be interfering with one another - what are the possible causes and can you recalibrate them in any way?

Basically I now have two compasses an old UK one and a new NZ one and they don't agree which way is north! Is it likely that the old one has been affected over time by other magnetic sources, or is it the case that the newer NZ one should not work here as I thought?

I'll double check both against friend's compasses at the weekend to see which one is closest but just curious.

Thanks in advance,
Graeme.

In reply to Graeme Seggie:

A compass will only work in one way, pointing to magnetic north, right? Which is surely the same place no matter where you are?

Okay, there's a fair bit of reversed polarity going on, bringing us closer to doomsday an' all, but it's still just a bit of metal that points north.

In reply to Graeme Seggie:
The magnetic field dips into the ground at different angles at different places in the world. This can cause compasses designed for one area to stick if used in another if the needle dips too much. You can buy worldwide compasses to deal with this, or slightly cheaper ones designed for various areas.
I can't think of anything that would cause compasses to actually point in different directions in the same place (unless they are completely reversed), but it could be that one of them is sticking, so not able to line up properly.

In reply to smithy:
yeah, that's what I would have thought as did the guy in NZ.

In reply to Graeme Seggie:
It is called I.C.E Individual compass error, so if your compass is out by a degree or so you have to compensated for this, ie like you would do for Grid to Mag add.

In reply to Big Smudge:

How do I know which is out though and what if both are?

In reply to Iain Forrest:

Yeah the only thing that should be different is what they call Magnetic Dip. I think it's DOWN in the Northern Hemisphere and UP in the Southern Hemisphere. Should both show North the same way though!

I think Magnetic North itself has moved about 900m over the last few hundred years since it was discovered becuase of the Polar Ice moving etc. Still doens't really change the old 3% rule though.

In reply to Graeme Seggie:

Get a map, and work out the bearing from one point to another.
Stand at one, take a bearing on the other.
See how much your bearing is out from what it should be.

Repeat for other bearings to see whether it's consistent or your compass is just sticking. If the former, you can correct for it; if the latter, use a different compass.

In reply to Iain Forrest:


It doesn't have to be completely reversed, just have the permanent magnetism in the needle disrupted a bit. The magnetised needle consists of a lot of molecular-sized magnets all oriented the same way. If enough of them get dis-oriented then the needle itself will no longer reliably point north. That can happen by exposure to a strong magentic field, especially a varying one, such as you might get near a large-ish electric motor for example.

In reply to Graeme Seggie:

By taking a bearing to a point you know on the map. Not forgetting magnetic variation.
Then using a Protractor workout the correct bearing from the map and take one from the other.

In reply to Jon Hemlock:

Okay - thanks for the previous reponses re dip. I just rechecked them in the same spot lined up against a floor board and they are closer together in direction than before when I had them apart but against the same floor board. Maybe there was a pipe behind the floor board close to one compass pulling it off alignment? That's the only thing I can think of anyway.

Thanks again.
Graeme.

In reply to Iain Forrest:

Okay - thanks for that.

In reply to Graeme Seggie: the following article may help - http://en.wikipedia.org/wiki/Compass#Compass_correction

I think the upshot is that compasses are designed to be used in differetn areas - so you can't use a compass, designed for use in New Zealand, in the Northern Hemisphere.

hope this helps.

In reply to Graeme Seggie:

You do realise that placing them close together will alter the readings? Same with having them close to electronic gear, the reading may not be true, then there are your ice axes as well.

In reply to smithy:

Not true, compasses work differently at different latitudes, you can buy, even in the UK compasses calibrated as such. There is an outdoor supplier, I cant remember its name, which will supply a compass for whever you are going in the world.
In actual practice, over short distances I dont think it makes a lot of difference.

In reply to Al Evans: Sorry, that should have said an outdoor supplier in Stockport.
Good to see you back Sutty.

In reply to Jon Hemlock: John the magnetic variation varies both ways, so its not just a one way shift.

In reply to Graeme Seggie: there are 5 zones of magnetic variation for balanced needle compasses,
all zone compasses are available from silva and suunto

Yes, I never had them close together when comparing but perhaps in my original comparison, one or both of them were close to metal sources behind the floor board they were lined up against. When separately used in the same spot they give the same reading so in retrospect now I think all is well.

In reply to Al Evans:

It would seem so...I'll have to go very quiet and try and sneak out of the forum backdoor unseen...

In reply to Joe Miller:


Good info there - cheers!

In reply to gear boy: used to be explained on silva site but found this instead which helps
http://casanovasadventures.com/catalog/compass/p1403.htm

In reply to Graeme Seggie:

Your floorboards - they wouldn't be held in place by steel nails would they..... Or electrical cables under the floor..........

Stand up and do it by eye

In reply to smithy:

Though to be fair Smithy, your assumption seems logical, can anybody explain why its not true? I know the fact, but not why.

In reply to Al Evans: I guess it could be due to the 'dip' thing, with a slightly higher/lower mount for the needle?

But, if all this IS so, howcome I've never heard of it before? I've owned Silva for years, and never heard of this, ever. Not denying it, just surprised my Pack Rat mind doesn't know about this.

TTG

In reply to Graeme Seggie:

There are two issues being confused here. Reading the links already given will clarify.

Declination: the fact that magnetic, polar and map north are all annoyingly in different places. Therefore, you need to offset your compass reading east/west accoring to where you are. This is nowt to do with the compass. Maps have declination info on them. Search your OS with care. It is on there.

Dip: this is nothing fancy. If you're stood at the north pole (magnetic), then your compass wants to point down. If you're at the south pole, the north is too far away to pull much, but the south pole will suck the south end of the needle downward (ie your compass wants to point up). Manufacturers build in a compensating tilt according to the area in which a compass will be sold. This stops the dip from making the needle stick.

Mark

In reply to O Mighty Tim:

Because you didn't read the manual? Or the map? Or a book on how to navigate?

Being cheeky really, but it's all ifno which is generally out there in relevant places.

In reply to O Mighty Tim:
It is. The Earth's magnetic field lines are vertical at the magnetic poles, level with the ground somewhere between the poles, and dipping into / out of it at various angles elsewhere.
Probably because the needle will still point North if it can move freely, it's just that it will dip - if it doesn't get stuck, the compass will still be right.