Few pieces of kit repay close attention like a compass. It weighs next to nothing, needs no batteries, and keeps working in weather that turns a phone screen into a grey smear. But a compass is only as good as the person holding it, and that person is only as good as the compass in their hand. A cheap model with a sluggish needle and no romer will fight you every time you try to take a bearing.
Baseplate compasses are the sensible default for walkers, climbers and anyone navigating from a paper map. They are not all alike, though. Here is what genuinely matters on a hillside, and what you can happily ignore.
The plate is the part you handle most, so it should feel right before you look at anything else. A clear acrylic baseplate lets you see the map underneath while you line the edge up with a grid line. Check that the edge is genuinely straight — run a fingernail along it — and that the corners are rounded enough not to catch on a map case or the inside of a pocket.
Length is a trade-off. A longer plate, roughly 12 cm or more, spans two grid squares on a 1:50,000 map and makes alignment far more accurate. A short plate disappears into a chest pocket but wobbles when you are trying to be precise. If you navigate mainly by map and compass rather than by phone, buy the longer one. Look too for a luminous direction-of-travel arrow, and a baseplate marked with a centimetre rule for measuring distance.
In Britain you will almost always be using Ordnance Survey or Harvey maps at 1:50,000 or 1:25,000, so those are the romer scales you want printed on the baseplate. A romer is simply a grid of tiny squares that lets you read a six-figure grid reference directly, without estimating by eye.
To use one, place the corner of the romer on the bottom-left corner of the grid square containing your point. Read the eastings along the bottom scale first, then the northings up the side. It takes a few seconds once you have done it twice, and it removes the guesswork that turns a rescue rendezvous into a wild goose chase.
Two-degree graduations on the bezel are the norm and quite readable. One-degree markings are finer, but harder to read in dim light or with cold hands.
Grid north and magnetic north are not the same direction. The difference is magnetic declination, and every decent map prints a diagram in the margin showing it, along with the annual change so you can update it for the map's age. In Britain the difference is small — often only a degree or so, and it varies from place to place — but a small error held over a kilometre or two of featureless ground puts you in the wrong re-entrant.
You have two ways to deal with it. The first is to remember the rule and add or subtract it each time, which works until you are tired and the cloud is down. The second is an adjustable declination screw on the compass itself, which lets you set the offset once and then take bearings straight from the map. That is genuinely useful, but only if the setting is correct: an adjustable compass that was set for a trip in another region months ago will quietly lie to you. Check it before you set off, not after.
Some compasses instead have a small declination scale inside the capsule, which you use as a temporary adjustment rather than a permanent one. That is a reasonable compromise, and it never goes out of sync without you noticing.
A mirror sighting compass has a folding mirror with a sighting line, letting you take a bearing on a distant object with the compass held at eye level. It doubles as a signalling mirror, and some models add a clinometer for measuring slope angle. If you do a lot of bearing work, night navigation or winter mountaineering, it is worth the extra bulk.
For most hillwalking and lowland navigation, a plain baseplate with a magnifier over the bezel and a small sighting notch is lighter, cheaper and perfectly adequate. The magnifier matters more than people expect: it is what lets you read the fine detail of a busy map without taking your gloves off.
A liquid-filled capsule settles the needle and stops it swinging. Good compasses settle in a second or two; poor ones leave you waiting while you drift off your bearing. A small bubble in the liquid is common and not usually fatal, but a large one means the capsule is leaking and the compass should go back.
If you travel outside Europe, check whether the needle is balanced for a magnetic zone or is a global needle. A compass balanced for one zone will drag against the capsule in another and give unreliable readings.
Buy the compass that suits the maps you actually carry and the way you actually walk. A large mirror compass is dead weight if it lives in a rucksack pocket and you navigate by thumbing a 1:25,000 map. A tiny keyring compass is useless if you are trying to take a precise bearing across a plateau.
If you can, handle the compass before buying. In the shop:
Then take it outside and practise. Set a bearing in the garden or a local park, walk fifty paces, and check where you arrive. Learn to set the declination, and recheck it whenever you pick up a map for an area you have not walked before. The features on the baseplate only earn their keep once you have used them enough that they stop being features and start being habit.
Photo: marcelkessler / Pixabay
