Will the accuracy of a compass be permanently degraded if it is stored for a long time in a ferrous metal container? I put a Cammenga wristwatch compass in a tin from BestGlide, saw the needle peg to the metal & wondered if that would screw up the compass over time.

I don't know about accuracy but I have had a compas totaly revers on me. OK that may seem inacurate but once you realize that you just put the white in the shed and not the "red in the shed" and you are Good to go.

But again that was a revers poloraization.

I wish I knew the definitive answer.

My suspicion is that proximity to metal won't hurt it. It is proximity to magnetic fields (from a magnet or other strong magnetic fields) that can hurt it.

I found this statement from Silva (the real one from Sweden, not the U.S. one):

"Never expose compasses to extreme temperatures (above + 60°C or below -40°C), or to magnetic fields such as knives, radio speakers, magnets etc. Such exposure can cause permanent damage to them."

Knives???

I found this http://www.zoom-one.com/navigati.htm

It says to avoid speakers, computers, and other electrical or electronic devices. It also says "Storing a compass in a hot car can also damage it because the liquid inside may expand, thus rupturing the seal and start leaking" and "If you want a compass to store in a car, you should get one that is designed to take very warm storage temperatures, or not use a damping liquid inside."

I'm no scientist but it doesn't seem like it should affect the magnetic quality of the compass needle. Where is Blast when we need him? If that were the case then simply using the compass would cause degradation.
IIRC don't magnets degrade slightly over time anyway? Again where is Blast when we need him.

Storing a compass near un-magnetized iron/steel should not be an issue. Magnets work by having all there atoms spinning in the same orientation (simplified model). Two things can scramble this uniformity, either physically striking it hard enough to knock all the atoms out of alignment or by introducing a conflicting magnetic field.

The conflicting magnetic field can come from either a permanent magnet or simply from an electrical charge passing through wires. A weak extrenal magnetic field from an electrical cord can be as damaging as strong magnetic field from a speaker, it just will take the weak field longer to de-align the compass atoms' spins.

Non-magnetized iron/steel will attract the compass's needle, but all the compasss atoms' spins will stay aligned with each other. However, depending on how the metal was worked/treated some of its atom's spins may have ended up aligned. I happen to have a compass on my desk. I've pulled out my Leatherman Kick and pointed it's pliers at the compass, causing the needle to spin around and point it's rear towards the LM. Switching the LM around causes the other end of the needle to swing towards the LM. This indicates that my LM does have some residual magnetism in it. I can't pick up a paperclip with it, but there's enough there to radically affect the compass.

Now I'm doing the same with a peice of N80-steel pipe. The compass needle spins to point at this peice of steel, but reorienting the steel in regards to the compass does not change the direction of the needle. In fact if I pull the steel away and approach the compass from "behind" the rear of the needle now points towards the same spot on the metal. This peice of pipe has no residual magnetism in it.

I could lay the compass on the steel for years without any damage/disorientation of the compass atoms'spin alignment because there is nothing "pushing" at their spins. If I set it on my LM Kick the magnetic field from it will push at the spins and slowly cause them to "wobble". Different atoms in the compass will feel different amounts of push due to both distance from the pushing field and inhomogenaity in the magnetic field. The end result will be a mishmash of spin directions in the compass needle and so it's magnetic field will no longer be strong enough to line up with the Earth's magnetic field.

Now if I set the compass on a strong permanent magnet something slightly different can occur. In an undamaged compass all the spins align parallel to the needle. If you leave the compass next to a strong magnet this external magnet can realign all the spins towards it. The spins will still all be aligned, just not parallel to the needle. Now the north and south poles of the needle aren't at the tips but rather somewhere else along the needle and these new "poles" will point north/south.

If you want I can go on and talk about why the production technique of the pipe removed any possible magnetism while making the LM Kick may have slightly magnetised it. If you really get me going I might start adding vector diagrams and stuff like that.

-Blast, on a Friday afternoon

Given that compass precision is a very relative thing, even more so for hand held models, I wouldn't worry about it. Magnetic declination can throw you off twenty degrees or more, twice that if you misremember the direction of the declination and apply the correction the wrong way.

A useful overview:
http://en.wikipedia.org/wiki/Magnetic_declination

Determine the declination in your area:
http://www.magnetic-declination.com/

Human error throws in a few more degrees plus or minus, and local conditions, like those magnetized brass plated steel buttons, are going to screw with accuracy even more.

The bad news is that using a hand-held compass, even with correction for declination, most people can't maintain anything closer than five degrees. But it doesn't make much practical difference because even though the readings are off they are fairly constant in their error. Local conditions, like that fire steel you keep in your breast pocket, are more worrying because they are far less consistent.

I have compasses that are thirty years old and have never taken any special care of them beyond not abusing the pivot by dropping them. They all still point well enough to use.

I'm pretty sure some early versions of Ritter's PSK were packed with the compass next to some metal, so that it didn't work while in the pack. At the time it was said that not only would it start working as soon as you took it out of the pack, but that no long term damage would come from it being stored so close to metal. (They changed the packing anyway.)

There was a thread here, but I got nowhere searching for it.

Normally a compass that is surrounded by metal during storage will be fine; it won’t be affected in anyway. Store it next to metal or electronic equipment you will have a problem in that it could affect its accuracy.

If you can find a location with a known magnetic bearing on a particular date you do a sum to determine the magnet variation for the present date. Mark this new bearing on something solid. Set the compass on top of this and see how many degrees or mils the compass is out. When you come to use the compass just add or minus the number of degrees the compass was out and you have your true magnetic bearing.

These days it is made a lot easier by the fact there are programs that will give you the exact bearing for your specific location on that day.

I believe modern high-end gps units have a compass calibration algorithm built into them. But, I am sure you could quite easily work out how to do it on the standard cheapo gps units that are often talked about on the forum.

Calculating magnetic declination from a GPS (some actually display it, but not all):

1. Use the GPS Setup tools to set the GPS to True North - as opposed to Magnetic North.

2. Use the GPS to get the True North bearning to any waypoint. For example, the bearing from home to the nearby park is 262 degrees with the GPS set to use True North.

3. Use the GPS Setup tools to change the GPS from True North to Magnetic North.

4. Use the GPS to get the Magnetic North bearing to the very same waypoint (make sure you don't move the GPS). For example, the bearing from my home to the nearby park is 265 with the GPS set to use Magnetic North.

5. Subtract the Magnetic North Bearing from the True North Bearing to get the magnetic decliantion. In equation form this is

MagDeclination = TrueNbearing - MagNbearing

For my example, (262 - 265) = -3.

6. If the magnetic declination is negative, this means the needle points to the west of true north. In my case the needle points 3 degrees west of true north.

If the magnetic declination is positive, this means the needle points to the east of true north.

In the U.S. the magnetic declination zero-line (actually called the agonic line) pretty much follows the Mississippi River.

EAST of the Mississippi River magnetic north is WEST of true north. This is NEGATIVE declination since magnetic north is -x degrees from true north. It is also called WEST declination.

WEST of the Mississippi River magnetic north is EAST of true north. This is POSITIVE declination since magnetic north is +x degrees from true north. It is also called EAST declination.

You may have heard of the phrase "West is Best and East is Least". The West/East direction refers to the direction of the declination, not the region of the U.S., and whether you add or subtract to make your non-declination-adjusted compass point to the right place. Best means you need to add and Least means you have to subtract.