Eight short lessons take you from reading an OS map to walking on a compass bearing in cloud. Each one has an animated diagram and a quick question. Work through in order, or jump to the bit you need.
0 of 7 practice questions done
Lesson 1
The Map and the Grid
Scale
OS Explorer maps are 1:25,000. One centimetre on the map is 25,000 cm (250 m) on the ground, so 4 cm is 1 km. OS Landranger maps are 1:50,000, so 2 cm is 1 km. Explorer shows more detail, such as paths and buildings, and is the scale OS suggests for walking.
Ordnance Survey maps are always printed with north at the top.
Map distance to real distance
The grid
Great Britain is split into 100 km squares, each with two letters (NY covers most of the Lake District). On both Explorer and Landranger maps, grid lines are drawn every 1 km.
Eastings are the vertical lines. Their numbers rise as you go east. Northings are the horizontal lines. Their numbers rise as you go north. Always give the easting first: along the corridor, then up the stairs.
- 4 figures name a 1 km square. Take the easting of the line on its left edge, then the northing of the line along its bottom edge:
2533. - 6 figures pinpoint 100 m. Split the square into tenths each way (by eye, or with the romer scale on your compass) and add the tenths after each pair:
258 334. - 8 figures pinpoint 10 m:
2585 3345. GPS units often show these. On a 1:25,000 map 100 m is only 4 mm, so six figures is the everyday choice.
For a real place, put the two grid letters first, for example NY 258 334.
Tap anywhere on the practice map.
This practice map is made up, so it has no grid letters.
You want to tell Mountain Rescue where you are. Which do you give first?
Easting first, then northing: along the corridor, then up the stairs. Swapping them gives a completely different place.
Lesson 2
Reading Contours
Contour lines join points of the same height above sea level. Imagine slicing a hill at equal heights and looking down on the slices from above.
- Interval. On OS Explorer maps the gap between contours is usually 5 m, or 10 m in some mountain areas. On Landranger it is 10 m. Some lines are drawn bolder and carry their height.
- Steepness. The closer together the lines, the steeper the slope. Lines far apart mean gentle ground.
- Which way is up? Contour numbers are printed so the top of the number faces uphill.
Valleys and spurs
Both show as V or U shapes in the contours. The trick is which way the V points. If the V points uphill, it's a valley, and there is often a stream in it. If the V points downhill, it's a spur: a nose of high ground sticking out.
A set of contour Vs point uphill, with a blue stream running through their tips. What is it?
Vs pointing uphill show a valley, and streams run down valleys. A spur's Vs point downhill.
Lesson 3
Setting the Map
A set (or orientated) map is turned so that what's on the map lines up with what's around you. A hill ahead of you on the ground is then ahead of you on the map. It is the simplest way to check you are going the right way.
- By eye. In good visibility, turn the map until the hills, valleys, paths or a road junction on it line up with the real ones.
- By compass. When you can't see features, put the compass on the map and turn the map until the red end of the needle points to the top of the map, parallel to the north-south grid lines. You don't need to set a bearing or adjust anything for this.
Keep the map set as you walk. Turn yourself and the map together at each change of direction, so the text may end up upside down. That's fine.
You set the map with your compass. Where should the red end of the needle point?
The needle points north, and OS maps have north at the top. So turn the map until the needle points to the top, parallel with the grid lines.
Lesson 4
Know Your Compass
A baseplate compass (the Silva type) has a clear plate with a turning dial on it. Tap a part to find it. Then try the compass: drag the outer ring to turn the housing, or drag the plate to turn yourself.
Challenge
Set 135° on the housing, then turn yourself until the red needle sits in the shed.
Keep the compass away from phones, magnetic clasps, metal belts and similar, which can pull the needle. A few places, such as the Cuillin on Skye, have magnetic rock that can affect it too.
Which part do you line up with the grid lines on the map?
The orienting lines go parallel to the north-south grid lines, with the orienting arrow pointing to the top of the map. Ignore the needle when you do this.
Lesson 5
Taking a Bearing from the Map
First, estimate the rough direction, such as "north-east". It catches the classic mistake of setting the compass the wrong way round. Then follow three steps.
Grid north, magnetic north and the angle between
The map's grid points to grid north. Your needle points to magnetic north, which moves slowly over time. The angle between them is the grid magnetic angle. On Explorer maps it is in the key. On Landranger it is drawn in the middle of the top margin, and also in the key. It is only correct for the year the map was printed, so the British Geological Survey's online calculator gives the current value.
For over 350 years magnetic north was west of grid north in Britain, which is where the old rhyme "grid to mag, add" came from. From 2014 magnetic north moved east of grid north, starting in the south-west, and the line where they match has been moving north across the country. Across most of Britain, including the main mountain areas, it is now east, so going from map to compass you subtract. The British Geological Survey's rhyme is "east is least, west is best": subtract for east, add for west.
Today the angle is small, often around 0 to 2 degrees, so many walkers ignore it for short legs. Over long distances in poor visibility, a couple of degrees adds up, so it's worth knowing how.
Grid bearing to compass bearing
Your map bearing is 128°. The grid magnetic angle is 2° east. What do you set on the compass?
Magnetic north is east of grid north, so subtract: 128 - 2 = 126°. East is least.
Lesson 6
Walking on a Bearing
- Hold the compass flat in front of you, direction of travel arrow pointing straight ahead, away from your body.
- Turn your whole body, not the housing, until the red end of the needle sits inside the orienting arrow: red in the shed.
- Look along the direction of travel arrow and pick a fixed object on that line: a boulder, a tree, a clump of heather. Walk to it, check again, pick the next one.
Don't stare at the needle as you walk. Most people drift to one side. Aiming at features between you and the target is far more accurate. In thick cloud, choose features as little as 25 m away, or send a companion ahead on the bearing and steer them left or right.
How far have I gone? Pacing and timing
Count double paces: every time your right foot (say) hits the ground. An average adult takes about 60 to 65 double paces per 100 m on flat ground, but yours will differ, so measure it. Walk a known 100 m at your normal pace a few times and take the average. Up or downhill, rough ground and snow change your count, and steep slopes shorten your stride a lot.
Timing is the other check. At 4 km/h you cover 100 m in 1.5 minutes. At 5 km/h it is 1.2 minutes. Naismith's rule adds 1 minute for every 10 m you climb.
Pace and time helper
On the hill: tap once each time you finish 100 m worth of paces.
You take 62 double paces per 100 m. How many for a 300 m leg on flat ground?
62 for each 100 m, three times: 62 × 3 = 186 double paces. Expect more on rough or steep ground.
Lesson 7
Navigation Techniques
Long bearings across open ground are hard to follow accurately, because most people drift. These techniques use the land to make the job easier. Pick one to see it in action.
A footbridge sits on a long river. You deliberately aim to the left of it. When you reach the river, which way do you turn?
That's aiming off. Because you aimed left on purpose, the bridge must be to your right when you hit the river.
Lesson 8
Final Quiz
Sources
- Ordnance Survey: Map reading made easy (scale, grid, north at the top, contour interval)
- Ordnance Survey: Understanding map scales
- Ordnance Survey: Beginner's guide to grid references
- Ordnance Survey in The Great Outdoors: Understanding grid references (6, 8 and 10 figures, romer)
- Ordnance Survey: Understanding map contour lines and Advanced contours and relief
- Ordnance Survey: Beginner's guide to using a compass
- Ordnance Survey: A tale of three norths, Magnetic north continues its march to the east and Three norths: where are they now?
- British Geological Survey: Something's happening to magnetic north in Great Britain ("east is least, west is best")
- British Geological Survey: The great north run and grid magnetic angle calculator
- grough: Is it time to ditch the old walkers' mnemonic?
- Silva compass manual (parts, the Silva 1-2-3 method)
- Duke of Edinburgh's Award: Navigation (compass parts, setting the map)
- Mountaineering Council of Scotland (now Mountaineering Scotland), navigation series: compass bearings, estimating distance, setting the map, aiming off, attack points, handrails, catching features, relocation
- British Mountaineering Council: A few navigation techniques
- Ordnance Survey: Advanced guide to measuring distance (pacing, timing)
- Glenmore Lodge: Navigation essentials
All maps and diagrams on this page are drawn for this course and are not Ordnance Survey mapping.