Because they rely on radio signals, GPS units generally do not work underground or underwater, but what about a plain ol' magnetic compass? Are the magnetic fields of earth present at all depths? Does water do anything to attenuate them?

Can I go spelunking with a compass to help me keep my bearings or even go through a train tunnel in a mountain (contrived example)? Do divers use compasses regularly?


2 Answers 2


Compasses are good equipment both spelunking and diving. Even the deepest cave you could go to is still near the earth's surface, geologically speaking. The earth's magnetic field is also essentially the same under water as above.

If you are using a compass, what you need to be aware of is nearby magnets and large sources of iron. So if you were exploring an underwater metal shipwreck, it could throw off a compass, the same as if you are testing a compass in your house standing next to a metal stove or refrigerator. The same is probably true if you explored a cave with a very concentrated source of iron ore. If you store your compass with a strong magnet, you risk demagnetizing it so it won't work at all. But if you treat it properly, a good magnetic compass is going to be more reliable than any electronic ones I've tried.

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    One thing to be wary of underground: you will probably need a light to read the compass, so make sure that you either don't use a light which affects the compass or ensure the light is always held far enough away not to affect the compass.
    – Paul Lydon
    Commented Apr 19, 2016 at 11:15
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    Structures will alter the earths magnetic field, these days they even use geomagnetic positioning technology to map interiors of steel structure buildings. Fortunate thing about navigating underground is the magnetic field is still the same as when the cave was mapped, so the readings will be the same when you go through.
    – ShemSeger
    Commented Apr 20, 2016 at 17:43
  • What she said :) And when spelunking I would assume you need a light in anycase to see, but in scuba diving it is also a good idea to take a light as you never know how dark it can get :)
    – AquaAlex
    Commented Aug 4, 2016 at 6:28

Magnetic compasses work well in caves (both dry and submerged), with the usual caveat that iron-rich minerals can cause deflections. In mines, consider using regular back-bearings for confirmation.

Compass bearings are the main source of data for cave surveys (along with distance and inclination or altitude measurements, of course). Cave surveyors use either a mechanical (needle) compass or an electronic one for data gathering, regularly achieving closures of better than 1%. I think the electronic ones use Hall Effect to measure the magnetic field direction.

In Britain, a wrist compass is considered standard equipment for diving.

  • How does one ensure there is no iron nearby?? Visual inspection will work only in very limited scenarios.
    – ahron
    Commented Feb 10, 2022 at 6:58
  • Usually by knowledge of the geology or dive site. Limestone caves are low in iron, whereas iron mines are quite obviously in an iron-rich deposit. Commented Feb 10, 2022 at 7:42
  • Caves are kinda specialized so I assume this sort of information is already documented, but if someone is going hiking, they usually don't know what minerals are in the surrounding hills.
    – ahron
    Commented Feb 13, 2022 at 13:46

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