How to use a compass
For those who enjoy rambling out in the unknown in search of wildlife a compass could be a lifesaving bit of kit, but even if you carry one do you know how to use it should you find yourself lost? Liz Beverley from Ordnance Survey passes on her tips for successfully using a compass.
Learning how to use a compass is an important outdoor skill; not just for walkers, but cyclists and runners as well as those who like to go off the beaten track in search of wildlife, too.
Why do you need a compass?
A compass helps you to find where you are and find your way; this is very useful but can be critical if you get lost and visibility is poor. The main ways you use a compass are:
- ‘Setting’ the map with the compass so that it matches what you see on the ground, and that you’re pointing in the right direction.
- Taking a bearing from the map and walking on a bearing (direction)
- Using a bearing to identify features on the ground.
Compasses come in many designs, but most compass features are the same. Before you use a compass to navigate you should familiarise yourself with the main features.
- Baseplate – the plastic base.
- Compass housing – or the compass wheel, with a mark every two degrees covering 360 degrees, and N-S-E-W (the ‘cardinal points’).
- Magnetic needle – red end for north, white for south.
- Compass lines – on the bottom of the baseplate (also called ‘orienting lines’.
- Orienting arrow – fixed and aligned to north.
- Index line – extension of the direction of travel arrow.
- Direction of travel arrow.
- Map scales 1:25 000, 1:50 000 and metric measurer (known as Romer scales).
The main working part of a compass is the magnetic ‘needle’ that floats on a central pivot. The red end always points to the Earth’s magnetic north pole and the outer ring is marked with the cardinal points of the compass (N-S-E-W) and every 2 degrees. These markings are used to get bearings (the direction from where you are, to where you want to go).
If you rotate the ring to line-up the red north of the needle to the red arrow on the baseplate, a bearing can be taken from the compass ring.
Where is north?
There is, however, a slight complication; magnetic north is not the same as map grid north because magnetic north (where the compass needle points) changes in different areas of the world, and also over time.
For a completely accurate reading you have to adjust the bearing to take account of the difference between map grid north and magnetic north. The degree of deviation is marked on printed Ordnance Survey maps but as a rough guide, you should adjust by 2 degrees by turning the compass housing anticlockwise.