Reading an OS Explorer Map: Contours, Symbols and Grid References

A good map is one of the quietest pleasures of walking. Unfolded on a gate or a drystone wall, it tells you what lies over the next ridge, where the path crosses the stream, and how long the climb is likely to last. The Ordnance Survey Explorer series is the one most walkers in Britain carry, and learning to read it properly is well worth an evening at the kitchen table.

Scale and what it means on the ground

Explorer maps are printed at 1:25,000, which means that four centimetres on paper represent one kilometre on the ground. That is a generous level of detail: field boundaries, individual buildings, small woods and many tracks are shown. The larger Landranger maps, at 1:50,000, cover a wider area with less detail and suit planning a long route or a drive. For walking, most people prefer the Explorer.

Before setting out, spend a minute with the scale bar and the key printed at the side. The symbols vary slightly between editions, so a quick look at the legend avoids guessing.

Contour lines: the shape of the land

Contours are the thin brown lines that join points of equal height. On an Explorer map they are usually drawn at five-metre intervals. Where the lines crowd together the slope is steep; where they spread out the ground is gentle. A set of closed rings with the highest number in the middle shows a hill or summit, while rings that open towards lower ground, with a V-shape pointing uphill, trace a stream valley.

A useful habit is to look at your route and count how many contour lines you will cross. Each line is five metres of climbing, so crossing ten of them means roughly fifty metres of ascent. It gives you a feel for effort that distance alone does not.

Paths, access land and symbols

Public rights of way are marked in green on Explorer maps, with different dash patterns for footpaths and bridleways. Areas of open access land are shaded with a yellow tint and bordered in orange. Other symbols show things such as car parks, viewpoints, youth hostels, churches with towers, and the location of springs and boundary stones. Blue marks water, from the thinnest stream to the open sea, while green shading shows woodland.

It is worth remembering that a map is a snapshot. Paths can be diverted and gates can change, so treat it as a guide rather than a guarantee. If you meet a waymark that disagrees with your map, trust what is in front of you and think carefully before continuing.

Grid references

The faint blue grid squares are one kilometre wide. Each is numbered along the edges, and a six-figure reference uses those numbers to place a point within a square of a hundred metres. The rule is to read along the corridor before going up the stairs: first the eastings, which run along the bottom, then the northings, which run up the side. Estimate the tenths of the square by eye. Being able to read out a position accurately is valuable if you ever need to explain where you are to someone else.

Orienting and using a compass

Turn the map until what you can see around you matches what is printed. Even without a compass this simple step makes the landscape much easier to understand. When you do carry a compass, it helps with taking bearings in poor visibility, so it is worth practising in clear weather first. For planning a full outing, our day walk checklist pairs well with a good map.

Frequently asked questions

Is a phone map enough? A phone app is handy, but batteries fail and screens crack in the rain. A paper map and the skill to read it make a reliable back-up.

Which map should I buy first? Choose the Explorer sheet that covers your usual walking area, then add others as your routes spread out.

How do I protect a paper map? A clear map case, or a folded map in a zip-lock bag, keeps it dry and still readable.