How to use a compass with a map, step by step, declination included

The plateau is crystal clear at the start. Then the cloud rolls up from the valley, the cairn opposite disappears and the trodden line fades into the short grass. That is when the compass stops being a gadget at the bottom of your rucksack and becomes the most useful thing you carry, provided you learnt to use it before you needed it.

Ordnance Survey puts it plainly: phones and mapping apps are hugely useful, but batteries run flat and signal drops, while a compass works anywhere [1]. It does not replace the map, it completes it: the map tells you where to go, the compass tells you which way.

A walking compass, part by part

The baseplate compass that walkers use comes down to five parts [1]. The clear baseplate, whose straight edge works as a ruler between two points. The magnetised needle, whose red end points to magnetic north. The rotating dial, or bezel, marked from 0 to 360°. The orienting lines and arrow in the bottom of the housing, which you line up with map north. And the direction-of-travel arrow on the baseplate, which shows the way to go.

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A bearinga direction measured clockwise from north, from 0 to 360°: 90° is east, 180° south, 270° west is simply a precise direction. You do not need to remember the numbers to start with: what matters is that the dial holds the direction once it is set [1].

Taking a bearing from the map, in four moves

1. Line up the two points. Lay the compass flat on the map with one edge of the baseplate running from where you are to where you are going, the direction-of-travel arrow pointing towards your destination.
2. Turn the dial to map north. Without moving the baseplate, turn the dial until its orienting lines run parallel to the north-south grid lines, orienting arrow to the top of the map. The bearing is set; ignore the needle for now.
3. Line up the needle. Lift the compass, hold it flat in front of you and turn your whole body until the red end of the needle sits inside the orienting arrow.
4. Follow the direction-of-travel arrow. Pick a fixed landmark far ahead along its line, a lone tree, a boulder, a summit, walk to it, then repeat.

These four moves are the ones Ordnance Survey teaches beginners, summed up in a phrase that has stuck: put red in the shed [1]. OS also flags the two most common mistakes: moving the dial once it is set, and not holding the compass straight out in front of you [1]. Aiming for a landmark rather than walking with your eyes on the needle also gives a truer line [1].

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The needle’s north is not the map’s north

There are three norths, and this is where beginners come unstuck [2]. True norththe direction of the North Pole, around which the Earth spins, the north of the meridians. Grid north, the north of the map’s grid. And magnetic north, the one the needle points to, which shifts slowly from year to year [2] [3]. The angle between true north and magnetic north is called magnetic declinationthe angle between true north and the north shown by a compass, measured east or west; also called magnetic variation; it varies with place and date [3]. Ordnance Survey adds a third angle that matters on its maps, the grid magnetic anglethe angle between grid north and magnetic north, printed on OS maps [2].

Across France, the national mapping agency describes declination as ‘almost zero in mainland France today’ [3]. We calculated it for 23 October 2026 with the World Magnetic Model, WMM2025, the model run by NOAA and the British Geological Survey [4] [5]: it is indeed almost nil in Brest, 0.05°, but reaches 3.2° east in Chamonix and 3.5° east in Strasbourg. At home, it is 1.2° west at Fort William and close to zero on Snowdon. Elsewhere it changes everything: 14.6° east at Mount Rainier, in Washington State, and 15.3° west at Mount Katahdin, in Maine [5].

Do you need to correct for it? On a path, with landmarks everywhere, an error of 2 or 3° changes nothing; Ordnance Survey says most beginners in Britain can ignore it, as variations here currently range from -1° to 4° [1]. On a featureless plateau or in the mist, the French mapping agency gives the order of magnitude: a 1° error on a point 6 km away puts you about 100 m out [3]. OS’s rule of thumb is ‘from grid to mag add, from mag to grid get rid’: you add the grid magnetic angle to a bearing taken from the map before following it, and subtract it from a compass bearing before plotting it [2]. Magnetic north has moved from west to east of grid north for the first time in Britain since the 1660s, starting in the far south-west and spreading across the country over 12 to 13 years, so the angle is turning negative in more places: check its sign on your map [2].

On OS Explorer maps the grid magnetic angle is in the legend, on Landranger maps in the north margin; it holds only at the date of printing, and the British Geological Survey’s calculator gives it to the day [2]. Walking in France? Since March 2014, the edge of IGN maps follows the projection rather than true north, up to 5.5° out near Brest: for precise sightings, line up with the meridians [3].

Practise before you need it

A compass is best learnt in good weather, on ground you know: that is Ordnance Survey’s advice, and I cannot think of better [1]. A familiar circuit, a bearing at every junction, and the moves become second nature before the day they count. To go further, an orienteering club or a guided navigation day teaches you to read the land and pick landmarks far better than any article. On waymarked paths, the compass mostly confirms the waymarks, which our guide to trail waymarks helps you read; and to know how long the way back will take, estimate your walking time.

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Worth watching

Ordnance Survey, whose four-move method the article follows, shows Steve Backshall taking a bearing from the map and turning it into a direction to walk.

The video is hosted by YouTube (Google): nothing loads until you play it, and Google may then set cookies.

Guillaume Hoarau

Camping, wild camping, bushcraft and hiking

Guillaume Hoarau writes for Outfarer about camping, wild camping, bushcraft and hiking. A survival-skills enthusiast and a collector of old field manuals, he likes clear explanations, lists that earn their place and other people’s stories, which he tells well.

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