How to Read a Map on Low-Visibility Walks

Before the cloud comes down

Low visibility rarely arrives without warning. The forecast will usually hint at it: a cloud base forecast below the summits, a warm front sliding in from the west, or a still, damp morning with mist pooled in the valleys. Reading that forecast properly is the first navigation skill of the day, because it decides whether you carry the map and compass in the top of your pack or in your hand.

Before you leave the car park or the village, do three things. Fold the map so the day's section is visible without unfolding the whole sheet, and keep it in a clear case so rain does not turn it to pulp. Mark your intended route, your escape routes and at least two obvious catching features — a road, a wall, a ridgeline — that would tell you if you had drifted. Then give someone at home your route, your estimated finish time and a time by which they should raise the alarm.

• A baseplate compass, checked against the map before you set off
• Waterproof map case and a spare map of the same area if the walk is long
• Headtorch with fresh batteries, even on a summer day
• Whistle, spare layer, and food you can eat without stopping

Setting a bearing you can actually walk

Lay the compass on the map with its long edge running from where you are to where you want to be. Rotate the bezel until the orienting lines inside it sit parallel with the north–south grid lines on the map, with the N on the bezel pointing to the top of the sheet. Read the number at the direction-of-travel arrow. That is your grid bearing.

Out on the hill, hold the compass flat at waist height, turn your whole body until the red end of the needle sits inside the red housing — "red in the bed" — and the direction-of-travel arrow now points along your line. Do not walk staring at the compass. Look up, pick a target twenty or thirty metres ahead that you cannot miss: a boulder, a lone tussock, a fence post. Walk to it, then repeat. This is slower than striding off into the murk, and considerably faster than walking back up the slope you have just descended in error.

In Britain the difference between grid north and magnetic north is currently small enough to ignore for most hill walking, but the margin note on your map will tell you the local variation and how fast it is changing. Get into the habit of glancing at it.

Pacing, timing and the quiet arithmetic

Distance in fog is a guess unless you count. The trick is double paces: every time your left foot lands, that is one. On level, rough ground most adults manage 60 to 75 double paces per 100 metres. Uphill the stride shortens, downhill it lengthens, and on deep heather or bog your count will drift. Calibrate yourself on a measured stretch of track before you rely on it.

For time, use the old rule of thumb: allow roughly one hour for every four kilometres, plus one minute for every ten metres of ascent. A five-kilometre leg with three hundred metres of climb works out at about an hour and three quarters, before you add stops. Write your timings down. A leg that should have taken forty minutes and has taken an hour is information, not bad luck.

Reading contours under your boots

Contours are the honest part of the map. On a 1:25,000 sheet they are usually at ten-metre intervals, and the closer they crowd together, the steeper the ground. Widely spaced lines mean gentle ground; a single line running across your path is barely a rise.

• Stand on a spur and the ground falls away on both sides; on the map the contour lines make a rounded nose pointing downhill
• In a re-entrant the ground rises on both sides; the contours form a V pointing uphill
• A convex slope feels like it drops away suddenly; concave ground pulls you in and hides the summit
• Rock hachures on the steep side of a contour mean crags, not just steep grass

Match what you feel with what is printed every few minutes, not once an hour. If you expected to be walking level across a hillside and your left knee is complaining about a sideways slope, the map will tell you which side of the spur you are actually on.

Handrails, attack points and aiming off

A handrail is any linear feature you can follow without compass work: a boundary wall, a stream, a forest edge, a ridgeline. Walking two hundred metres to a wall and then following it is often quicker and far safer than a bearing across open moor.

Aiming off is the navigator's insurance. If you want a bridge on a stream, deliberately aim a few hundred metres to the left of it. When you meet the water you know precisely which way to turn. Aim dead at the bridge and you may cross the stream without noticing, or arrive unsure whether you are upstream or down.

An attack point is a large, unmistakable feature close to your objective — a junction, a cairn, a bend in a track. Navigate carefully to the attack point, then take a fresh bearing for the final short leg.

Knowing when to stop walking

Set a turnaround time before you leave and keep to it. Cold, wet and tired all erode judgement long before they erode your legs, and the walker who pushes on "just to the top" is the one who ends up benighted.

If you think you are lost, stop. Eat something. Put on a layer. Take the map out, orient it, and find one feature you can identify with certainty — a contour shape, a wall corner, the sound of running water. Build from there. Turning back early is not a failure of nerve; it is the skill that keeps you walking for decades.