An Ordnance Survey (OS) map is a detailed, accurate map of Great Britain produced by the national mapping agency, Ordnance Survey. To read one, you need to understand four key skills: identifying symbols from the key, using the scale to measure distances, giving and locating grid references, and interpreting contour lines to understand relief (the shape of the land).
What is an Ordnance Survey map?
Ordnance Survey has been mapping Great Britain since 1791 and produces maps at a range of scales for different purposes. At KS3, the most commonly used scales are:
- 1:50,000 (Landranger series, pink cover) — 2 cm on the map = 1 km on the ground. Good for planning routes across a wide area.
- 1:25,000 (Explorer series, orange cover) — 4 cm on the map = 1 km on the ground. More detailed; better for walking and fieldwork.
OS maps use standardised symbols and colours across every sheet. Roads are shown in different colours depending on their class (motorways are blue, A-roads yellow, B-roads brown). Woodland appears as green clusters of tree symbols. Buildings are grey, churches with towers have a particular symbol, and footpaths are shown by dashed lines.
The key (or legend) explains every symbol used on that map sheet. Always read the key before trying to interpret an unfamiliar map.
How to use the scale
The scale tells you the relationship between distance on the map and distance in the real world.
On a 1:50,000 map: 1 cm on the map = 50,000 cm in reality = 500 m = 0.5 km. Therefore, 2 cm = 1 km.
To measure a straight-line distance:
- Place a ruler between your two points on the map.
- Note the measurement in centimetres.
- Multiply by the scale factor (e.g. on a 1:50,000 map, multiply cm by 0.5 to get km).
To measure a curved route (e.g. a road or river): Place a piece of string along the route, mark the total length, then measure the string against the scale bar (the graduated line printed on the map margin). The scale bar is the most accurate method because it accounts for any distortion in the map printing.
How to give a four-figure grid reference
OS maps are divided by a grid of blue lines — eastings (running north-south, numbered left to right) and northings (running west-east, numbered bottom to top).
A four-figure grid reference identifies a 1 km square on the map. The rule is: "along the corridor, then up the stairs" — read the easting first, then the northing.
Example: Grid reference 3452 means the square that begins at easting 34 and northing 52.
Step by step:
- Find the easting (the vertical line on the left edge of your square). Read the two-digit number printed at the bottom of the map.
- Find the northing (the horizontal line at the bottom edge of your square). Read the two-digit number on the side of the map.
- Write them as a four-digit number: easting then northing.
How to give a six-figure grid reference
A six-figure grid reference narrows the location to a 100 m square, making it far more precise. It is the standard used for locating specific features such as a church, a spot height or a viewpoint.
Each 1 km grid square is imagined to be divided into tenths both horizontally and vertically. You estimate (or measure) how many tenths across and how many tenths up your feature sits within its 1 km square, and add these as third digits to each component.
Example: If a church sits roughly three-tenths across and seven-tenths up within the grid square 3452, its six-figure reference is 343527.
A useful memory aid: "I went along the corridor (eastings) to flat number 3, then up the stairs (northings) to floor 5 — then added the decimals: 3 and 7."
How to read contour lines
Contour lines are brown lines on an OS map that connect all points at the same height above sea level. Every contour line on a 1:50,000 OS map represents a 10 metre vertical interval (on 1:25,000 maps it is also 10 m, with index contours every 50 m shown in brown with the height printed on them).
Key rules for contour lines:
Close together = steep slope. When contour lines are packed tightly, the ground rises sharply. Cliffs are shown by contours merging or by a specific cliff symbol.
Wide apart = gentle slope. Well-spaced contour lines indicate a gradual incline or flat land.
V-shape pointing uphill = valley or river. Rivers flow downhill, so they cut valleys. The V of the contour lines always points upstream (towards higher ground).
V-shape pointing downhill = ridge or spur. A ridge of high land jutting downhill produces V-shaped contours pointing away from the high ground.
Closed oval or circle = hill summit. If the innermost contour is a complete closed loop, it marks a summit. The spot height (▲) printed inside or nearby gives the exact elevation.
Worked example: interpreting a contour pattern
Imagine a KS3 exam showing contours numbered 40, 50, 60, 70 and 80 m, with the lines packed closely on the east side of a hill and spread out on the west. This tells you:
- The eastern slope is steep — good for showing processes like mass movement or rapid river flow.
- The western slope is gentle — more suitable for farming or settlement.
- There is a summit at or above 80 m.
- If a river symbol snakes through the contours in a V-pattern on the east, it confirms a steep-sided valley — in geography, a characteristic of upland or youthful river landscapes.
This kind of inference — reading the map and then drawing geographical conclusions from it — is exactly what KS3 and GCSE exam questions ask students to do.
Frequently asked questions
What does OS stand for in geography?
OS stands for Ordnance Survey, the national mapping agency for Great Britain. Founded in 1791 originally for military purposes (the name reflects this — "ordnance" refers to military artillery and equipment), it now produces maps for leisure, planning, education and emergency services. Its most well-known series for walkers and students are the 1:50,000 Landranger maps and the 1:25,000 Explorer maps.
How do you remember which way round to read a grid reference?
Use the phrase "along the corridor, then up the stairs." You read the easting (horizontal position) first, because you walk along the corridor before going up. Then you read the northing (vertical position), because you go up the stairs. This applies to both four-figure and six-figure grid references — easting always comes before northing.
What does a spot height mean on an OS map?
A spot height is a precise height measurement, shown as a small black dot with a number (e.g. ▲ 342) printed on the map. It gives the height above sea level in metres at that exact point, usually a summit, road junction or benchmark. Spot heights are more precise than reading a height from contour lines because they are surveyed directly. On 1:50,000 maps, triangulation pillars (trig points, shown as a blue triangle symbol) also carry height values and were used to build the original survey network.
What is the difference between a four-figure and six-figure grid reference?
A four-figure grid reference identifies a 1 km square and is accurate enough for locating a village, a lake or a general area. A six-figure grid reference narrows this to a 100 m square by estimating position within the 1 km square — it is used for locating a specific building, a viewpoint, a campsite or any individual feature. In examinations, six-figure references are expected whenever you are asked to locate a precise point on the map.
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