Choosing a compass: baseplate, mirror and sighting compass

The compass needle is magnetised and aligns itself with the Earth's magnetic field. It points to the magnetic north pole, which does not exactly match the geographic north pole. The angle between the two directions is called declination (or magnetic variation). It differs from place to place and changes slowly over time. National agencies offer calculators to find the current declination for your location.

The Earth's magnetic field is not horizontal everywhere. Towards the poles it tilts more and more towards the ground. A simple needle would therefore hang at an angle in some regions and scrape against the housing. Manufacturers compensate for this by balancing the needle for particular regions of the world. So-called global needles are built to work worldwide.

The housing is filled with liquid. It damps the needle's movement so that it doesn't swing back and forth for long but settles quickly. This makes reading faster and more accurate, especially when you hold the compass in your hand.

The baseplate compass is the basic tool for most hikers. It consists of a transparent plate with a direction-of-travel arrow, a rotating housing with a degree scale, orienting lines in the bottom of the housing and rulers along the edge. With it you can measure a direction on the map and then follow it in the terrain, or the other way round.

The mirror compass is basically a baseplate compass with a folding lid that holds a mirror and a sighting notch. When you raise the compass to eye level, you see the target through the notch and the needle in the mirror at the same time. This makes bearings to distant targets more precise. The lid also protects the housing.

The sighting or lensatic compass has a sight for aiming and a small magnifying lens for reading the scale. It is robust and common in military use; a US field manual describes it in detail. For map work it is somewhat more awkward without a transparent baseplate, because you cannot transfer directions directly.

Look for a clearly readable scale. Usually it is divided into degrees; a full circle has 360 degrees. Some compasses are divided into mils, which are mainly common in military use. Which unit you choose matters less than that map, compass and everyone in your group use the same one.

The rulers on the edge should match the scales of your maps so you can measure distances directly. Adjustable declination is handy if declination is large in your area – you set it once and then don't have to calculate every time. Luminous markings help at dusk.

The needle matters too. If you travel abroad, check whether your needle is balanced for that region, or take a compass with a global needle. Toy or keyring compasses are not suitable for serious navigation; they are often inaccurate and not damped.

Metal, magnets, phones, speakers and some magnetic clasps on jackets or bags deflect the needle. Even a phone in your chest pocket can distort the reading. Keep away from such things when taking a bearing, including cars, fences and power lines. Also don't store the compass right next to electronics or strong magnets, because in unfavourable cases these can weaken the needle's magnetisation.

Test a compass before you rely on it. Reasons to replace it are bubbles in the housing, a sluggish or sticking needle and a reading that differs clearly from a second compass. Such a comparison is easy: lay both compasses side by side, a little apart, and see whether they point in the same direction.

Don't rely only on the compass in your phone. It needs power, can be inaccurate if not calibrated and fails along with the phone. A real compass works without a battery and is a good backup – also in a go bag.

A compass is only as useful as the practice you have with it. Start in a familiar area: lay the compass on the map, orient the map to north and compare what you see with what the map shows. Then take a bearing on a church tower or a summit and find it on the map.

The actual navigation – taking a bearing, allowing for declination, holding a course – is explained in detail in the navigation world. This entry only helps you choose the tool. Many clubs and adult education centres offer courses where you learn map and compass together with others.