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Compass magnetic zones

Do compasses work anywhere?

As an example let’s say you’re heading off on a trekking holiday to The Inca Trail (Peru) or Kilimanjaro (Tanzania), will your compass, bought in the UK, give you accurate bearings whilst on your trip?

The simple answer is that most won’t, but a few (more expensive) models will.

The reason for this is that to work correctly the needle needs to rotate freely around it’s bearing and not touch either the top or bottom of the inside of the dial.

Normally this free movement can be achieved by simply ensuring that you hold the compass level when using it. However the earth’s magnetic field, which the needle follows, is not the same everywhere. In some places it flows (almost) parallel to the ground, in other places (further from the equator) it tilts downwards towards the ground.

You can imagine it this way. Let’s say that the Earth contains an extremely large magnet a few thousand miles underground. As you know your compass points towards the North (magnetic) Pole. If you are in the northern hemisphere this means your compass needle will point north. But, if you are in the Southern hemisphere your compass needle will actually point towards the end of the Earth’s, imaginary, internal magnet, which is actually very deep underground. So your compass needle will dip and so not freely rotate.

To compensate for this change in inclination, of the magnetic field, good quality compasses have extra mass added to certain sections of the needle and the bearing which the needle rotates about. This balances the needle and allows it to rotate freely.

The major manufacturers e.g. Silva, Suunto, etc. all make (more expensive) compasses which will work virtually anywhere.

Silva separate the world in to five magnetic zones and sell compasses designed to work in each

Suunto have only two zones

It’s important to note that there are no “industry standards” with regards to magnetic zones and each manufacturer decides on their own zoning. This can cause a few problems. For example a Suunto designed for use in Northern Brazil is simply not going to function correctly on the South Island of New Zealand or Antarctica, the Silva zones appear to be based on not only the magnetic variations but also national distribution systems, etc. But at least they have recognised that there is a problem and have attempted to solve it. 

So, if you are only intending to use your compass in the country you bought it that’s fine. But, it you are setting off on a trip or expedition you should either get a compass when you arrive or ensure you buy one of the global models.