Understanding OS Map Symbols: A Quick-Reference Guide

Ordnance Survey maps pack an enormous amount of information into a small space, and most of it is conveyed through a fairly small set of consistent symbols and line styles. Once you know what you're looking at, an OS map tells you not just where a path goes but how steep the ground is, what kind of right of way you're following, and where you can legally be at all. This is a practical run-through of the symbols that matter most for anyone planning a walk or a wild camp.

Contour lines: reading the shape of the land

Contour lines are the thin orange or brown lines that snake across the map, each one joining points of equal height above sea level. On the 1:25,000 Explorer series they're usually spaced at 5-metre intervals (10 metres on 1:50,000 Landranger maps), with a thicker line and a printed height value every so often to help you keep track. The single most useful thing to know is that spacing tells you steepness: contour lines packed tightly together mean the ground rises or falls sharply, while widely spaced lines mean gentle, easy terrain. A slope where the lines are so close they almost merge into a solid band is worth taking seriously — that's a crag or a very steep scramble, not a stroll.

Water and woodland

Blue is used consistently for anything water-related: rivers, streams, lakes, reservoirs and the sea are all shown in blue, with watercourses as blue lines of varying thickness depending on size. Green shading marks woodland, with different symbols distinguishing coniferous, deciduous and mixed or orchard planting — useful both for navigation (a wood is a solid handrail feature) and for knowing where you'll lose sight of landmarks and any phone signal.

Rights of way: footpath, bridleway or byway

This is the distinction that actually affects where you're legally allowed to walk, cycle or ride, and OS maps show it through consistent line styles on the Explorer (1:25,000) series:

• Public footpath — a short green dashed line. Walkers only.
• Public bridleway — a green dashed line with longer dashes than a footpath. Open to walkers, horse riders and cyclists.
• Byway open to all traffic (BOAT) — shown with a green "+" style marking, and open to walkers, riders, cyclists and, in principle, motor vehicles.
• Black dashed line — a path or track that is not a public right of way. It may be a permissive path, a private track, or simply a feature on the ground, but you have no automatic legal right to use it.

On the older-style 1:50,000 Landranger maps the same rights of way are shown in pink rather than green, so it's worth checking the legend on whichever map you're actually holding rather than assuming colour conventions carry across both series.

Boundaries and other symbols worth knowing

Administrative boundaries — county, parish, or national park boundaries — are typically marked with distinct dotted or dash-dot lines, usually labelled in the legend rather than colour-coded in a way you can guess at a glance, so it's worth checking the map key for the specific boundary type shown. Point features use recognisable pictograms: a small cross or a church-with-tower/spire symbol marks places of worship, triangulation pillars are shown as a small blue triangle, and car parks, viewpoints, and campsites all have their own dedicated symbols listed in the map's legend. The legend printed on every OS map is the definitive reference — symbol conventions are consistent across a given map series, but it's always worth a glance at the key for anything you're not immediately sure of.

Grid references: pinpointing a location

The blue grid lines across an OS map are the National Grid, and they let you describe any point on the map with a short string of numbers instead of a description. Each grid line is numbered along the map's edges. The standard method is remembered as "along the corridor, then up the stairs": read the number of the vertical grid line to the left of your point first (moving along), then the number of the horizontal grid line below your point (moving up), and combine them.

For a basic 4-figure reference, take the two-digit numbers of the grid lines to the left of and below your point and put them together — this identifies the 1km grid square your point falls within, accurate to within a kilometre. For a more precise 6-figure grid reference — the standard used for mountain rescue call-outs and precise route planning — estimate tenths of the way across that square in each direction and add a third digit to each half: so instead of "34, 72" you might get "342, 728", combined into a single six-figure reference "342728". This narrows your position down to a 100-metre square, which is precise enough to describe a specific gate, summit, or bothy. A 6-figure reference is genuinely useful safety information to know how to give — it's often the fastest way for mountain rescue or a walking partner to find you on a large, featureless hillside.

The bottom line

You don't need to memorise every symbol on an OS legend before your first walk, but the handful covered here — contour spacing, water and woodland shading, the footpath/bridleway/byway distinction, and how to build a 6-figure grid reference — cover most of what you'll actually use on the ground. Keep the map's own legend to hand for anything unfamiliar, and the rest becomes second nature with a bit of practice.