I’m going to write a series of short articles instructing on the use of a compass during expeditions. Article number one focuses on the compass itself.

How does a compass work?
A compass is a magnetised needle that aligns itself with the Earth’s magnetic field. Since this field runs approximately from the north pole to the south pole we can quickly determine in which direction north, and therefore any other direction, lays. The compass allows us to determine directions without reference to any geographical features. This becomes very handy when you’re lost in low visibility.

What compass do I need?
There are many types of compasses which although work on the same principle, are designed to be more useful for different tasks. The most useful type for expeditions is the “base plate” compass. The base plate compass consists of a transparent rotating fluid filled capsule in which the needle lays, mounted on a transparent plastic base. The fluid filled capsule causes the movement of the needle to settle quickly whilst the rotation and transparent base plate allows for easy measurement of bearings.

A basic but perfectly adequate base-plate compass should only cost you £10 from any good outdoors store. Silva is probably the best known make and most people actually refer to “base plate” compasses as Silva compasses. I use a 25 year old compass made by Freiberger Präzisionsmechanik so there’s no problem with using older or second hand compasses as long as they still work!

Finding your way around the compass


The needle:The needle of the compass always points north. On this particular compass the ‘point’ of the needle is indicated with a small blob but on most compasses the ‘point’ will be the red end of the compass.

The housing:The compass housing rotates. As it rotates the compass will continue to point north. The red lines in the middle of the compass will rotate with the housing. On the edge of the housing the four compass points are marked along with a scale on which bearings can be read. Make sure that the scale can be accurately read on your compass. The letters are wearing off on my compass through use but the small markings for bearings are still clear. The fluid in my compass has also discoloured over 25 years, expect a new compass to be clear.

The base plate:The base plate features lines running parallel to it’s length. These are to help in aligning the base plate with a bearing. The left hand side of this compass features a scale on which distances can be read on a 1:25,000 scale map. Although not essential this is a very useful feature. A 1:50,000 scale is slightly less useful on most expeditions.

The string:If your compass doesn’t have any string, you can’t hang it around your neck. and you’ll be forever dropping it or forgetting which pocket you left it in. Non-elastic string is also useful for measuring distances on maps. Whilst not technically essential I wouldn’t consider a compass without string.

The rest:This compass is slightly more advanced than a basic model. It features a magnifying glass for inspecting details on maps (useful for splinters too!), centimeter graduates for measuring map features, a click counter for pacing or lap counting and ‘glow in the dark’ markings for night navigation. None of these features are at all essential for any expeditions so don’t worry if your compass lacks them but it gives you some idea of what you’ll get with a more expensive model.