Compass Questions

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phatch

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Oct 3, 1998
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I've got two questions.

First, in my search for my kit components, I've come across compasses advertised as made for particular locals. I'm assuming this is some sort of built in declination adjustment, but I'm not sure. Some say they're just for parts of North America others for just the Norhtern hemisphere and that you need a different compass if you different places. What's up with that?

Second is on packing a compass into a kit. There's just no where appreciably distant from iron and steel. a knife, a zipper, a wire saw, you know, all that ferrous metal. How far does a compass need to be stored from these items in a kit to maintain acceptable accuracy?

Phil
 
As to your first question, a compass' regional range has to do with the way Earth's magnetic field is shaped.

Imagine a bar magnet with its two ends, north and south. You have probably seen the trick where a bar magnet is laid on a surface covered with iron filings, and the filings line up to form curves from one end (pole) of the magnet to the other.

Earth's field is like this too, with lines of magnetic force going from pole to pole (actually, the magnetic poles are beneath the surface of the Earth quite a ways). Our compasses line up with these lines of force, which can be used to find direction. You already knew that.

The weird part, that few people realize, is that the lines of force are not horizontal everywhere. In the northern hemisphere they point north, but they also plunge down into the ground. In the southern hemisphere they come up out of the ground (convention says lines go south to north). The field lines are approximately horizontal at the equator, and are vertical at the north and south magnetic poles.

That is why you can't get a compass reading at either magnetic pole -- the lines are straight up and down, but a directional compass needs at least some horizontal pull. You can buy mining compasses that will actually measure the vertical angle.

Here's a page with a pic or two:

http://liftoff.msfc.nasa.gov/academy/space/mag_field.html


Basically, the compass needle has to balance horizontally on its jewel so that it can swing. In the northern hemisphere, the S end needs to be heavier so it won't stick up in the air; the N end needs to be heavier down south. It is really hard to make a compass needle that can handle a large range of latitude.

As to your second question, I know that you can shield magnetic things with iron, but I don't know the details. maybe someone with more electrical knowledge can give a good answer there.

Scott
 
PS:

I did some searching and found that most magnetic shielding is made of alloys with a lot of nickel. Apparently iron is very "permeable" for magnetic field lines, so is used to direct and channel them. Nickel is very impermeable.

You could build a box for a compass, or avoid magnetic kit items. A lot of stainless steels are not magnetic, for example.

Another alternative is to use a compass that is resistant to field disturbance. My military lensatic is extraordinarily resistant to extraneous fields, and should be just fine packed next to a knife.

Scott
 
I knew the earth's field varied a lot, thus map declination, but the bit about the heavier south end for us norhterners was interesting.

In reading a bit at equipped.org, their opinion was that while the compass would be screwy packed up, that once away from metal it would work appropriately. Caveat being that not all compasses return to normal very fast. My friend has a button compass that can reportedly take from 1-24 hours to return to normal if it's kept level and away from metal for that time. Not very useful IMHO.

Thanks for the info.

Edited to Add, declination also adjusts for magnetic north not being true norht, but also vor magnetic field vagaries as some areas have wierd magnetic distortions.

Phil
 
Yes magnetics are weird and if you saw the recent excellent PBS program you'd know that the poles are going to reverse again, it's already starting.
 
Magnetic pole reversals happen every so often and are not predictable. The Earth's magnetic field functions as about 90% dipole (like a bar magnet) and the other 10% hard to model. The 10% signal helps the poles to undergo "excursions" where they wander around, generally within about 10°-12° of the rotational axis. This is why there is a declination correction for any given locality.

The last I heard from the geophysics literature, leading opinions wre for quadropoles or octopoles. When the magnetic field (the dipole) reverses, it does not sweep to the other end. It actually shuts down for a few thousand years and then starts back up. This is a chaotic process and we can only model it statistically.

The average compass will only work well to about 50° north and south of the equator because the mount for the needle gets inefficient as the inclination of the field approaches vertical. Compasses are available with international mounts that will work within a few degrees of the poles. I have a Brunton pocket transit that I have used up to 65°15" north with a declination of 38°E and 85° inclination. As long as the compass is not near large amounts of iron when you use it you are OK. you don't have to worry about what is in your pack with it.

Declination changes by location as I mentioned above. The USGS has a calculator online that will give you the current declination and inclination if you enter your location, the date and your elevation WRT sea level.

http://www.ngdc.noaa.gov/cgi-bin/seg/gmag/fldsnth1.pl

The last reversal happened a few hundred thousand years ago, and statistically could happen again at any time. There have been times when the field didn't reverse for millions of years eg. the Cretaceous long normal magnetic period.
 
IMHO the easy way to choose a compass is to go out and buy one that you can set the declination yourself, that way you can't go wrong.

As already stated the magnetic field is not straight, cause the earth is curved. Also when you use a compass it will point to magnetic north and not true north. The declination is the difference between the two. and it varies depending on where you are. There is a formula that you use but I don't remember it anymore. It also changes year to year. If you have a map it'll tell you what the change of declination is, then you use the formula to figure it out for your area.(I'll look it up in my notes)

As for storing it in your bag. It Depends on the type of bag being used ie. Back pack or military ruck style. I've always had a military ruck ( was in Army Cadets) ready to go. I kept my Silva Compass in the outter middle most pocket when I wasn't using the ruck, and I kept my knives and other metallic goodies on the outter sides or in the main or top pouch. When I was using it I kept it in my breast pocket or around my neck. I never had a problem with it going screwey on me (unless I was surrounded by Iron Pirate). Then again it's a $50 compass.

So, IMHO go out and buy a decent compass either in degrees or mils then take a course on map reading or get a buddy to help you out.
 
I'd be interested in some actual brand name recommendations of a good compass.
 
Fine compasses are to be had from Brunton/Nexus, Suunto, and Silva.

The original Silva (Sweden) markets their compasses here in the States under the Nexus brand. The Silva compasses marketed in the USA are actually made by Suunto. (Confusing enough?)

In any case, my own preferences would be to go with any of the "Ranger" styles offered by the above companies (all are fine compasses) as well as the newer Brunton Eclipse (#8099).
 
The optical compasses are by far the most accurate and fastest out there. I use a Suunto which is accurate to 1/4 degree. You hold it up to one eye, with both eyes open. One eye sees the target, and the other sees an aiming line and a magnified azimuth ring. Brunton also makes a similar compass.

The Brunton Eclipses are a great improvement to the standard mirror compasses.

The military lensatics are also very good -- tough as nails and pretty fast to read. They are heavy, though.

I have found really accurate compasses like the above to be best for actual navigating. If you stay on the trails you can get by with a zipper-pull model.

Scott
 
Brunto pocket transits are calibrated to 1°, but you can pretty much get into the fractions if you're careful. I have used them mapping professionally for years with no problem. They have the added advantage, that you can repair them in the field with a SAK or Leatherman, if you have spare parts.

Using USGS 7 1/2' topo maps and most compasses, you can get within 10m accuracy easily if there is any amount of topographic relief

There are a few high tech alternatives out there at several times the Brunton $200+ price. If you need the extra accuracy. Most who need this kind of precision use either GPS units or laser transits these days.
 
I like the mil-spec cammenega compasses. I've been disappointed with the consumer grade ones too many times.
Bob
 
If it´s for normal outdoor use, I believe a simple map compass from Silva or Brunton would suit you fine. I feel that a declination compensation feature is nice to have but not a necessity, you should learn about declination anyway and be able to use a compass without the feature.

Don´t use the compass while it is inside a steel container or in the handle of a survival knife, etc., hold it in your had a foot or so away and it will work OK.

A good page on using a compass:

http://www.learn-orienteering.org/old/
 
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