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General Gear Discussion The General Gear Discussion forum is for the discussion of traditional and lightweight (ultralight) backpacking gear that is not covered in other Practical Backpacking™ forums. [Please post about Backpacks, Shelters, Sleeping Gear, Backcountry Kitchen (Food, Stoves) in those respective forum areas.]


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  #1  
Old 07-02-2008, 04:52 PM
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Cossoft Cossoft is offline
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Help with a compass that works worldwide

Some people might think it strange that I'm asking this question, but I need to replace my compass.

I've never really noticed, but do compasses work worldwide? If I buy one in Britain, will it work properly in South America, Oz etc? Or do I need a 'Global Compass?' I had thought of going for a Silva Expedition 4 till I read Global Compass info.

Do they really only work in particular world zones? And if so, what's the point of a global compass then? Does anyone have any advice?
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  #2  
Old 07-02-2008, 06:05 PM
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Rocketman Rocketman is offline
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Quote:
Originally Posted by Cossoft
I've never really noticed, but do compasses work worldwide? If I buy one in Britain, will it work properly in South America, Oz etc? Or do I need a 'Global Compass?' I had thought of going for a Silva Expedition 4 till I read Global Compass info.


Do they really only work in particular world zones?


A compass is basically a magnet on a swivel which aligns itself with the "North" component of the local magnetic field. The magnetic North Pole is thousands of miles awy from the true North Pole, and thus almost everywhere on Earth, the compass does not point to North correctly. You can find maps that show where magnetic North is at the present time. It wanders a bit.

They call the local deviation between the "Compass North" and "true north" the magnetic declention or something similar. You can find maps of what the current magnetic corrections are for any point on the glove on the internet. I have forgotten where.

Where I live, the magnetic error is only a few degrees. In California it was 17 to 20 degrees East - the compass pointed East of true North.

There are some places where the error is nearly 180 degrees.... way up near the north pole - and the compass points to some place a few thousand miles away.

Quote:
And if so, what's the point of a global compass then? Does anyone have any advice?

Yes. Do some reading or just forget about intelligent use of a map and compass.

For a fashion accessory, there are some very nice jeweled compasses.

The Earth is round. The magnetic field isn't round but is some other funny shape, so some places the needle will not only point East or West of the true North, but it will point up or down as well. If you hold the compass level, depending upon where you are, the needle may point down so much that it drags on the baseplate and won''t accurately point at anything.

They build compasses that compensate for this. You could learn to compensate by not holding the compass level so the needle doesn't drag.

But some compass designs are set up for optical sighting, and that works best if the compass can be held level.

If you aren't going to do any fancy optical sighting, one of those little floating ball compasses will freely rotate everywhere on the planet Earth. But you can't do optical sighting. And the plastic ball compass will only point to local magnetic north. But, otherwise, it would be truly global.

Accuracy generally scales with the size of the compass. Those little plastic ball compasses aren't particularly accurate, and a big plastic ball compass is hard to put into a pants pocket.

Reviewing Earth geography might be a good thing.

Jim
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  #3  
Old 07-02-2008, 06:16 PM
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Surveyor Surveyor is offline
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Brunton makes a pocket transit (geologist's compass) that has an international suspension. This compass is supposed to be able to be used worldwide without having the to rebalance the needle.

The difference between magnetic and true north is called the declination and the difference can be substantial depending on one's location on the Earth. The Brunton I use has the capability to correct for the declination.

Hope this helps.
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Old 07-02-2008, 06:23 PM
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Cossoft Cossoft is offline
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Jim, those ball compasses - do you mean those that some people have suckered onto their dashboards?
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  #5  
Old 07-03-2008, 04:33 AM
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Rocketman Rocketman is offline
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Quote:
Originally Posted by Cossoft
Jim, those ball compasses - do you mean those that some people have suckered onto their dashboards?

Similar to those. You can get them on keychains as well. Being little, they won't be very accurate, much like a short pistol is much less accurate than a long rifle.

But you can use them worldwide.

You might want to consider an electronic compass, but my experience with them is just so-so from a practical useage. However, I have only used the electronic compass in an altimeter watch, and the dedicated compasses may be better. No mechanical needle to "stick".
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  #6  
Old 07-14-2008, 07:36 AM
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Cossoft Cossoft is offline
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Okay - I'm a little surprised. Is it really the case that armies, explorers and shot down pilots use those ball compasses..? You do not usually see them in camping outfitters, rather the more traditional ones like Silva.

Does this mean that they have already been selected by the shop for that particular part of the world? So if I buy for example the Expedition 4 in England, it won't work properly in New Zealand?

Does anyone have any experiences in different countries? Should I not bother with it all and just get a shiny new one?
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Old 07-15-2008, 12:39 AM
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David David is offline
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Correct me if I'm wrong, but I believe Silva does balance their compasses for use on the northern or southern hemisphere (that doesn't mean the compass won't work in the "wrong" hemisphere, but the needle will tend to dip at one end). The voyager series however can be used in both northern and southern hemisphere.
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  #8  
Old 07-15-2008, 05:04 PM
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Cossoft Cossoft is offline
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Thanks David - its the dipping that I was worried about. I'm going to do some tramping in New Zealand which is pretty southern hemisphere. Do you think that dipping is all that happens? So it's not really a big deal then?

I'm minded to play it safe though and have found a Sunnto M3G which is meant to work world-wide.
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  #9  
Old 07-23-2011, 07:24 AM
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Bushwalker Bushwalker is offline
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Quote:
Originally Posted by David
Correct me if I'm wrong, but I believe Silva does balance their compasses for use on the northern or southern hemisphere (that doesn't mean the compass won't work in the "wrong" hemisphere, but the needle will tend to dip at one end). The voyager series however can be used in both northern and southern hemisphere.
TRUE.

Read the labels on the packaging with any of the decent brand-name compasses (e.g. Silva, Brunton, Suunto..), and you will notice that they may be specified as being balanced either for the northern hemisphere or southern hemisphere, or it might be a universal model that handles both and can be used nearly anywhere..

Also note that as you approach both the North and South poles, both your normal compasses and most standard GPS units become more-and-more erratic and error-prone. For this reason polar travellers use special compasses and GPS models once they get within 40˚ (that's above 140˚N, or below 140˚S lat.) of their target.

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  #10  
Old 07-23-2011, 08:21 AM
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Ralph Ralph is offline
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The accuracy of a compass is a function of the card size (or ball diameter). Maritime and aviation compasses are relatively large and so can be read to less than 1 degree. The compass I use is a Dakar lensatic that combines the features of a baseplate compass (for map applications) and a sighting compass (for mapping and navigation). The card diameter is about 2" but the magnifier (the "lens" in lensatic) allows me to eaily read to 2 degrees and interpolate to 1 degree. The card is suspended high enough that it is a global compass unless you are standing very close to the magnetic pole (not very likely for most of us). The compass folds to a little more than 2" x 3" and is liquid filled. It is also very reasonably priced at around $25. About the only flaw, if you want to call it that, is that the Dakar has no luminous feature for night use. A small red LED (red light does not affect night vision) is useful for that purpose and can also illuminate a map.

There is no mechanical declination adjustment but it's easy enough to add the declination to your reading (and if you travel a lot, setting a mechanical declination is a nuisance since it changes as you change position).

Actually, when using a map while traveling I usually use the compass to get a rough bearing and adjust the map by inspection. When moving I use magnetic readings almost exclusively. The compass I use when moving is a Suunto M-9 wrist compass. I get the magnnetic bearing, rotate the bezel to that and keep the needle between the bearing points. I can only read to 5 degrees with the Suunto but that is close enough - travel in the wilderness isn't flying through the air or sailing on the water, you are constantly making adjustments because of obstructions so the best you can do is approximate your direction.

Since I got out of the army I have almost never needed to navigate at night so that's not much of an issue.
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