What can you do with a compass?

A compass is a bit like an iPhone in that it is one device with many uses. When Steve Jobs introduced the iPhone to the world, he said he was about to unveil a computer, phone and camera.

Well, the compass has been around for a lot longer, and the best versions available today combine:

  • a needle pointing north
  • rulers to measure distances on the map
  • a protractor for measuring angles
  • a magnifying glass for reading the fine detail on maps

 

Let’s take these features one by one:

The Needle

Holding the compass flat, the needle will always point north, aligning itself with the earth’s magnetic field. This is known as magnetic north; there are actually three different norths. If you want to find out about them, click here, but you do not need to be concerned about them in the UK currently. 

What you do need to know is that the needle will be affected if it is near a magnet. Not only will it not point north, but prolonged exposure could result in something called ‘reverse polarity’, permanently changing the direction in which the needle points. There is a very strong magnet in your phone, so keep the compass well away from it.

Rulers & Scales

At the top of a compass like the Silva 4/54 (pictured above) there are three different rulers called  ‘Romers’ . A Romer is a way to measure distance on a map without having to resort to maths. Simply find out the scale of the map (normally 1:25,000, 1:40,000 or 1:50,000) and connect any two points on the map using the appropriate ruler. Then read off the distance on the ruler, which is numbered every 200m.

It is Romer, incidentally, not ‘roamer’. The principle of a ruler adjusted to the scale of a map was devised by Lieutenant Carrol Romer of the British Army in 1916 to make reading the distances on army maps easier.

This being said, you can also use the centimetre ruler on the left side of the compass to measure distance, but you need to use some maths. 1mm on a 1:25,000 map is 25m on the ground, 40m on a 1:40,000 map and 50m on a 1:50,000 map. So 17mm on a 1:25,000 map is 17 x 25m, in other words 425m on the ground. You can understand why the Romer is preferred by most people.

It should be pointed out that measuring distances on maps is of little use unless you can measure distances on the ground. Click here to learn about how to do this using pacing and timing.

A protractor

The dial with the numbers on it, known as the rotating housing or bezel, turns. So, if you line up the left or right side of the compass with a path on a map, then turn the dial so that the big red arrow (‘orienting arrow’) in the middle of the dial points to north on the map (ie, the top), you have created the angle between north and your chosen path. You don’t need to know the degrees, just the shape. All you now have to do is line up the big red arrow with (magnetic) north and you can be sure that you are heading off in the right direction. You do this by turning the compass (NOT THE DIAL!!) until the needle is over the big red arrow underneath, which some people think looks like a shed. This is called ‘putting red in the shed’. The arrow above the magnifying glass, known as the direction of travel arrow, will now point in the direction you need to take. This is one of the key things we practise on the Navigation in a Nutshell course.

A magnifying glass

There is a wealth of small detail on Ordnance Survey and Harvey maps. Kinks in contour lines, footpaths beneath rights of way… the magnifying glass towards the top of the compass can really help you identify these things.