Silva Compass problem

[Fitz]

I've got 2 identical silva compasses except one is weighted for South America the other Australia. I cant figure out which is which now! So will it matter if I use the South American one here or does anyone know how to distinguish betwwen the two

[whiskeylover]

Correct me if I'm wrong but I think you would need to maybe ask a surveyor about this. Hand held compasses are fairly inaccurate anyway aside from the fact that local magnetic anomalies will affect them, so it probably depends how very accurate you want to be. I have never heard of needing to get a different compass depending what country you are in. Maybe you should take both and try using them in different areas on good visibility days and using triangulation to known points, test their accuracy.
Please note I may be wrong but I don't believe it would make a big difference which one you use - test it to be sure - Tassie has some places where the accuracy of compasses is severely tested by magnetic anomalies in our geology (e.g Mt ironstone) Other metals in the vicinity - even barbwire fences can affect your accuracy.

[BarryJ]

Does it matter? I thought compasses were weighted for use in either the Northern or Southern hemispheres and therefore a compass for use in either South America or Australia would be the same.

The quote below is taken from this web site: http://www.williams.edu/Biology/Faculty ... mpass.html

An interesting detail is that there are northern- and southern-hemisphere compasses. This has to do with the fact that the magnetic field lines, to which a compass needle aligns, point into the earth at the north and south magnetic poles. In the northern hemisphere the north end of the needle is pulled downwards, and the south end is counterweighted to balance the needle. When you use a northern hemisphere compass in, say, Australia, the south end of the magnet is pulled downwards by the magnetic field, and is also heavier than the north end - resulting in a needle that catches and drags on the bottom of the compass housing when the compass is held horizontal.

Last edited by BarryJ on Mon 27 Oct, 2008 9:01 pm, edited 1 time in total.

[geoskid]

I read about this recently, see
(http://www.bushwalking.org.au/FAQ/FAQ_N ... tm#Compass)

[Fitz]

Does it matter? I thought compasses were weighted for use in either the Northern or Southern hemispheres and therefore a compass for use in either South America or Australia would be the same.

That was my impression too..maybe I was sucked in by some marketing. I cannot remember where I read the article that let me to buy the SA weighted one.
Yes I think I'll take them both and just see how they compare.
Thanks
Fitz

[rucksack]

Most compasses are balanced for different zones ... Australia is Zone 5 and South America spans three zones, 2, 3 & 4 and yes it does make a difference. A compass balanced for Zone 2 will not be accurate in Zone 5 and vice versa. You can buy global compasses (such as a Silva Voyager 8010) and use it in any zone, but the construction of the compass is different. You can read more about this at http://www.labsafety.com/refinfo/ezfacts/ezf313.htm or http://www.mapworld.co.nz/global.html or Google 'compass zones' or 'global compasses' for a more detailed explanation and to see the zones.

[Fitz]

Ok thanks, so I wsnt going mad when I bought it. But how to tell the diffenence now?

[whiskeylover]

I suppose if you try them in the field the SA one will 'drag' ? Other than that it would probably be hard to tell the difference as they both point to magnetic north still. It's just that the dragging of the needle in the housing might affect the accuracy of the SA one.

[Son of a Beach]

I always thought that it was just a matter of how close you are to the magnetic pole and if it's the north or the south magnetic pole, which affects how much the other end of the needle is weighted?

I would have expected that at least part of South America would be the same as at least part of Australia in this respect. However, rucksack (and others) have shown me the error of my ways. ie, the magnetic field does not line up with latitude anywhere near as closely as I thought it would (even taking into account the difference between magnetic poles and axis of rotation).

The two coloured maps of the earth's magnetic field (including rucksack's zones) shown here under the sub-heading 'However' are what helped me get this straight (or bent?). Thanks geoskid for that link.

[BarryJ]

I am like you, Nik. Until last night, I thought it was just a Northern/Southern hemisphere thing but (thanks to rucksack) I too am now much the wiser.

[Fitz]

A few years back I paid a visit to a Ukranian research base on the Antarctic peninsula. On of the "projects" they were doing was mapping the flux lines of the earths magnetic field and the effects of electric power generation on them. They were able to demonstrate that as a power station in the US came on line or boosted output the flux lines would be altered. They could tell by that stage which US power stations were up and running by the "look"
of the flux map! They could also tell that power generators other than those public ones were around by the effects they had on the flux lines when the were running- and they were able to roughly locate them. Interesting stuff!

[rcaffin]

I used the Silva I bought in Australia when we were in France. Worked just fine - until I lost it.
So i bought a cheap oil-filled compass in France - and use it most weekends here with no problems.

Yeah, theory says ... but practical experience says it doesn't matter. (As outlined in the FAQ someone referenced.)

Cheers

[photohiker]

I used the Silva I bought in Australia when we were in France. Worked just fine - until I lost it.
So i bought a cheap oil-filled compass in France - and use it most weekends here with no problems.

Yeah, theory says ... but practical experience says it doesn't matter. (As outlined in the FAQ someone referenced.)

Cheers

Oh dear. I'm going up against a guru here!

I've got two Silva compasses, one bought in Adelaide, and the other bought in Inverness. Here in Adelaide, the Inverness compass drags the south end of the needle on the bottom of the chamber and it doesn't swing freely until the north end of the compass is elevated to around 15 degrees. Whilst you could use it in a pinch, the risk is that you get a bad bearing with it and its also a PITA to use like that.

Michael

[davidmorr]

A few years ago, I took my Australian compass to Europe with me. The thing I noticed most was that the needle was tilted much more than here. The only way I could get it to work was to tilt the base plate until it lined up with the needle. Then it worked fine. The tilt required was about 30 degrees. However, I was not taking bearings with it which would have been difficult.

[sailfish]

Just put the 2 compasses on a level surface. The one where the needle or disc is level and swings freely is the one to use wherever you are.

Ken

[norts]

http://www.bushwalking.org.au/FAQ/FAQ_N ... emispheres

this explains the problem

Roger

[north-north-west]

*shrug*
My diving compass works just fine everywhere. But bulky for bushwalking, but.

[Orion]

I'm with Roger on this one. I bought a simple compass in Hobart fearing there would be enough of a difference to render my California compass less effective. But back home I notice that they are basically the same. Yes, one end of the needle hangs lower than the other and can even "drag", but it still works fine. And this difference is only apparent when I place them on a horizontal surface, something that is hard to do in the field. When hand held neither needle is usually level.

Maybe it's more important if you have a really expensive compass? I stick with simple models, around $15.

[photohiker]

I think there are several factors that lead to whether this is becomes a problem or not.

No-one is disagreeing that the needles tilt depending on where on earth the compass is, but some of us have found it to be a problem and others not.

I can see that a compass bought in Asia, for instance would have a smaller inclination difference to one bought in the UK. In my case, I bought the Inverness compass at Latitude 57N and the Adelaide one at 34S. In the linked article, there is a link to a map of the earth's magnetic "Main Field Inclination" Inverness is around +70 degrees (positive) Inclination and Adelaide is around -68 degrees (negative), a difference of 138 degrees. Most of Asia is in the range of 0 to +40. California is around +62

Another factor would be the dimensions and construction of the compass itself. The length of the needle and the internal height of the chamber would be the things to look at.

My Inverness Silva (Ranger 3) has a 33mm needle and an 8mm chamber depth (assuming a 1mm chamber thickness) while my Adelaide Silva (Voyager 8010) has a 29mm needle and a 9mm chamber depth.

Just looking at when the needles float mid-chamber and when they touch the chamber wall, there appears to be 4-5mm more tolerance in the Adelaide compass because of the differences in it's chamber and needle dimensions. (I'm sure a Pythagoras expert will be along shortly to verify this)

As far as I can tell, neither Silva has oil in it. I could be wrong on this, bit the needles move too quickly (and rattle when shaken) compared to oil-filled compasses I've had.

Neither of these compasses were expensive. I think I paid $30-40 for the Adelaide one a few years ago, and a good bit less for the Inverness one this year.

[Orion]

I took a look at my two compasses last night. One is a Silva "Starter" or something like that, purchased in California for about $15. The other has no marking for the manufacturer. It's a basic unit that I bought in Hobart last year for $15-20.

First I set them down next to each other. They pointed 30 degrees out of synch! Okay, I moved them away from the computer. Then they read the same. Same vertical tilt as well. Not even a hint of drag from the Hobart model. There is a huge difference in the magnetic inclination between the US and Tasmania. The Hobart model should drag, right?

So what's up? Did they sell me a northern hemisphere compass in Hobart? That seems very unlikely. Besides, I did use it down there a few times and it was fine.

Here's my educated guess. Upon close inspection both compasses use a pivot point that is above the center of mass for the needle. This is particularly true for the Hobart model which has a pronounced metal bell shaped section at the pivot. What this means is that any deviation from gravitational balance will induce a restoring torque, just like with a pendulum. If this torque is significantly larger than the magnetic torque then you won't see much dip. A back of the envelope calculation suggests this is feasible to engineer.

So, if my speculation is correct then why don't they do this with all compasses? I think there is probably a tradeoff in accuracy. The heavier needle will have a slower response and more friction; and consequently a larger dead zone, the region around the heading where friction overwhelms the diminishing torque. This is possibly supported by the fact that the less expensive Suunto models come balanced for only two zones whereas the higher precision units they sell are balanced for five.

[jasaj54]

Did you know...
Compass needles are weighted for different regions to counteract the downward magnetic deflection exerted on the needle, which may cause the needle to stick in the compass housing. Australia and New Zealand are in the Magnetic South hemisphere region, so the correct compasses for use here have the MS suffix. So, if you are travelling to another magnetic region, your Aussie-bought compass may not work!

[GPSGuided]

Then, there are compasses for global use, engineered to work in both hemispheres.

[jasaj54]

Correct...

[north-north-west]

Did you know...
Compass needles are weighted for different regions to counteract the downward magnetic deflection exerted on the needle, which may cause the needle to stick in the compass housing. Australia and New Zealand are in the Magnetic South hemisphere region, so the correct compasses for use here have the MS suffix. So, if you are travelling to another magnetic region, your Aussie-bought compass may not work!

It's only going over this thread has made me think - I've used my diving compass in a number of places in the northern hemisphere without problems, so presumably it's a global unit. I wonder if that's standard with the underwater compasses . . .

[GPSGuided]

How much did you pay? I suspect it comes with cost and quality. I also assume marine compasses are made for global application. One other, is your diving compass of the needle design or another? The particular design eg. Ball compass, are not impacted in the same way as for a needle compass.

[north-north-west]

Suunto, needle, but deeper unit than your standard map compass. Have two (or possibly three). One that went OS is around 25 years old (the other/s older) but can't recall how much it cost. It wasn't the cheapest around, but not as expensive as decent (surface) compasses seem these days.

[Bushman_Craig]

For "normal" trips I carry and use a Suunto MC-2G (a global compass). My other more modern compasses (Cammenga lensatic and Silva Ranger 3 respectively) are all balanced for Australia.

However, I also have quite a collection of vintage military marching compasses including a 1918 Verner's Pattern VIII prismatic and a 1943 MkIII prismatic and these work just fine here in Australia and are at least as accurate as my modern baseplate compasses. There isn't even any appreciable "dip" despite the fact that one of these old compasses came to me out of the UK and the other out of Canada.

Where do these fit in when it comes to Zones and Hemispheres? Has our modern technology gone backwards?

[Drew]

This discussion didn't seem to reach a firm conclusion, so I'll open the can of worms and ask the question: Should I just use my Australian compass (basic model Suunto) on my hikes in Peru next month, or buy a new one over there?

I guess I can take the Aussie one and if it drags a lot and seems annoying when I get there I can buy another. I'm sure I'll pay a premium in a tourist centre like Cusco though!

[neilmny]

Not a definitive answer but.....your Aussie compass is for the southern hemisphere and Peru is in the southern hemisphere. It has to be better than a northern hemisphere compass.

[Gadgetgeek]

As far as I understand it, it has a lot to do with the size and strength of the magnet, style of pivot, and all kinds of other factors.
Some manufactures list up to 5 zones for some compasses, maybe more in specialized cases, but according to suunto, they only list two, so north and south should work fine. If you are in doubt, you should be able to find an angle where the needle roams free, and with care doesn't touch the housing, that should give you at least a reasonable north estimate.
http://www.suunto.com/en-AU/Support/Compasses-feature-index/Understanding-balancing-zones/

If in doubt, I'd email them with a photo of the compass and see if they had any other thoughts on the matter.