How a compass works: Earth's magnetic field and declination

Earth's magnetic field is created deep inside, in the liquid outer core. Molten, iron-rich metal moves there, and these flows generate electric currents and with them a magnetic field. To a good approximation the Earth behaves like a large bar magnet whose axis is tilted slightly relative to the Earth's axis of rotation.

A magnet always has two poles, and opposite poles attract. The compass needle is magnetised and is turned by the field until it lies along the field lines. A curious detail: because opposite poles attract, the Earth's pole in the north is, physically speaking, a magnetic south pole – it attracts the north end of the needle. In practice this makes no difference, and people still speak of the magnetic north pole.

The field lines do not run horizontally but are inclined to the Earth's surface; near the magnetic poles they point steeply downwards. This tilt is called inclination or dip. A freely suspended needle would therefore tip down at one end. Compass needles are thus balanced for particular world zones.

On maps, north usually points to the geographic North Pole that the Earth spins around. The compass needle, by contrast, points along the local field lines, roughly towards the magnetic pole. The angle between the two directions is declination (also called variation). Depending on location, the needle points somewhat east or west of true north, and there are places where the difference is almost zero.

Because the flows in the Earth's core change, the magnetic pole wanders too, and declination changes slowly from year to year. That is why maps always give the year next to the value. With an old map you should check the current value, for example with an official calculator such as the one from the US agency NOAA, which computes the declination for a location from magnetic field models.

Small angles sound harmless but add up over distance. An error of just one degree means a sideways deviation of about 17 metres after one kilometre, and about 170 metres after ten kilometres. With a declination of several degrees, you can therefore clearly miss your target if you do not correct for it.

Hold the compass level when taking a bearing so that the needle can swing freely and does not drag on the housing. Keep metal and electronics at a distance: phone, headlamp, knife, belt buckles, but also cars, fences, steel railings and power lines. Anyone taking a bearing next to a car can easily get a wrong direction.

The terrain itself can interfere too. Some rocks contain magnetic minerals and deflect the needle locally. If bearings at one spot seem odd, it helps to walk a few steps further and check again, or to compare the direction with terrain features on the map.

When travelling to other continents, it is worth checking which world zone the needle is balanced for. If it does not match, the needle can tilt because of the different inclination and drag. There are also compasses with needles suitable for several regions.

Phones and watches often have an electronic compass. It measures the magnetic field with small sensors and needs calibrating; cases with built-in magnets can disturb it. It is handy but depends on the battery. A classic compass works without power or network.

In thick fog, on plateaus without landmarks or in forest where you can hardly see far, the compass is particularly valuable. There, a lot depends on declination being correctly accounted for and no source of interference being nearby. How to take and transfer a bearing in practice is covered in the navigation entries.

Place your compass on a table and slowly move a phone, a bunch of keys or a knife towards it. You will see at what distance the needle starts to swing – a simple experiment that shows how much distance is needed. You can try the same outdoors next to a car or a metal fence.

Look up the declination and its year on your hiking map and compare the value with a current official calculator. Then practise on a familiar, safe route transferring a direction from the map to the terrain, once with and once without the correction, and see how far you miss the target.