Why a Grid Reference Is the Most Useful Thing in Your Rucksack

Ask most people where they last swam and they will say "the little tarn under the crag, past the second beck". Ask a mountain rescue team and they will want eight characters and a clear voice. That gap is worth closing before your boots hit the path. A grid reference is simply a way of describing a spot on a map precisely enough that someone else can walk straight to it, or fly to it, without asking a single follow-up question.

On a remote tarn day this matters more than on a valley walk. Visibility can drop from twenty miles to twenty metres in the time it takes to eat a sandwich, and a shapeless expanse of grass and rock all looks alike. If you have written down where you started, where you are going and where you would head if things turned, you have a plan that survives bad weather rather than one that depends on clear views.

Reading the Numbers

British grid references are based on the National Grid. Each reference begins with two letters, then a string of digits. For northern Britain you will mostly be dealing with NY and SD across the Lake District and northern Pennines, SH for Snowdonia, and NN for the Scottish Highlands. Write the letters down – they are part of the reference, not decoration.

The digits come in pairs: an easting first, then a northing. The easy rhythm is "along the corridor, then up the stairs" – eastings run left to right along the bottom of the map, northings run bottom to top. On a Landranger at 1:50,000, the numbers along the edge go up in single kilometres, and the grid lines are spaced one kilometre apart. Explorer maps at 1:25,000 show the same squares with more detail inside them, which is why they are the better choice for tarn-hunting and wild swimming.

• Four figures – a one-kilometre square. Useful for a car park, not much use for a swim spot.
• Six figures – a 100-metre square. This is the everyday standard: good for a tarn, a path junction or a bothy.
• Eight figures – a 10-metre square. Overkill for walking, ideal if you are being winched into a helicopter.

To estimate the third digit of each pair, split the square in your head into ten strips. It is easier than it sounds after a couple of attempts, and nobody expects perfection. Say "NY 226 087" rather than "about NY 2260" – the listener needs to know how many figures they are hearing.

The Route Card: Start, Tarns, Escape Routes

Before leaving the car, write a short route card on paper and keep it in a map case or a trouser pocket. Paper does not run out of battery, and it survives a soaking better than you might think.

• Start point. The exact car park, lay-by or gate, with its grid reference and the time you set off.
• Each tarn you plan to visit. One reference per swim, plus a note on which side the approach comes from. A tarn at the foot of a crag is a very different proposition from a tarn sitting on a broad shoulder.
• Escape routes. At least two, each with its own reference where it leaves your route. Think valley paths, forest tracks, a bridleway running down to a road, or a descent to a hamlet.
• Cut-off times. A reasonable day pace on rough ground is three kilometres an hour, plus roughly a minute for every ten metres of ascent. Add a time by which you will turn around if you have not reached the tarn, whatever the weather.
• Nearest road access. Where a vehicle could realistically reach you, and its reference.

Read the card out to someone at home, and leave a copy. Telling a partner "I am doing a tarn in the Lakes" gives them nothing to work with if the afternoon goes wrong.

Grid References for Swimmers

A swim spot needs a slightly different reference from a summit. You want the point you will enter and leave the water, not the middle of the tarn, because that is where your clothes, boots and bag will be. Note the bank you plan to use and whether it is gravel, rock or squelchy peat.

Add a second reference for your changing spot if it is out of sight of the water, and a third for your first escape from the water – the nearest shore you could reach quickly if the wind got up or you started to chill. Cold water is the real constraint on a high tarn, not distance. Wind coming down a fell can turn a calm sheet of water into something choppy in minutes, so pick an entry with options on more than one side.

Swim in a small group, enter gradually, keep the session short, and leave everything as you found it. A remote tarn is often someone's drinking water further down the valley.

Phones, Paper and Saying It Out Loud

Mobile signal on northern fells is patchy and unreliable, and a waterproof phone case is not a substitute for a map. Carry the map, carry the compass, and treat the phone as a bonus.

If you do need help, give the grid reference slowly, letters first, then the figures, and say how many figures there are. Ask the operator to read it back. Add your location in plain words as well – the name of the tarn, the crag above it, the valley you are in. A reference and a description together are far stronger than either alone. Keep the phone warm inside your jacket, conserve battery, and stay put if you can safely do so.

Practise at home: pick three tarns on your map and write down their references without looking at the answer key. It takes ten minutes and it is the sort of small habit that turns a good day out into a day you can talk about for years.