Cartographic Skills » OS Maps: Grid References, Scale and Distance

Ordnance Survey maps

Ordnance Survey (OS) maps are the official detailed maps of Great Britain. They show the real shape of the land, roads, rivers, buildings and land use. In your exam you may be given an OS map extract and asked to use it. The skills in this lesson are the ones you need to find places and measure on the map.

There are two scales you must know. They show different amounts of detail.

Key terms:

• Ordnance Survey (OS) map: an accurate, detailed map of Great Britain made by the national mapping agency.
• Scale: the link between a distance on the map and the real distance on the ground.
• Grid reference: a set of numbers that gives the exact position of a place on an OS map.

Four figure grid references

OS maps are covered by a grid of blue lines. Each grid square is 1 km by 1 km on the ground. The lines have two-digit numbers printed along the edges of the map.

• Eastings are the vertical lines. They are numbered from west to east, left to right.
• Northings are the horizontal lines. They are numbered from south to north, bottom to top.

A four figure grid reference names a whole grid square. Follow two steps:

1. Find the easting on the left side of the square and write it down first.
2. Find the northing on the bottom of the square and write it down second.

A handy memory trick is "along the corridor, then up the stairs". If the easting is 34 and the northing is 71, the grid reference is 3471.

Six figure grid references

A four figure reference only tells you the square. A six figure grid reference pins down a spot inside the square, to within about 100 m. To do this, imagine each square is cut into ten equal strips each way.

1. Write the easting number of the square, then add a third digit for how many tenths the place is across the square from the left (0 to 9).
2. Write the northing number of the square, then add a sixth digit for how many tenths the place is up the square from the bottom (0 to 9).

Always give six figures as one number with no spaces or commas. The first three digits are the easting and the last three are the northing. To find a place from a reference, split it into two halves and do the steps backwards.

Scale and distance

Scale is shown on the map as a ratio, such as 1:50 000, and as a scale line (a ruler printed on the map). The ratio 1:50 000 means that 1 cm on the map is 50 000 cm on the ground. That is 500 m, so 2 cm is 1 km.

Remember that the same real distance looks twice as long on a 1:25 000 map as on a 1:50 000 map. Always check which scale you have before you convert.

Compass direction

OS maps are drawn with north at the top. Direction is given using the points of the compass. There are eight points to know.

• The four main points: north (N), east (E), south (S) and west (W).
• The four in between: north east (NE), south east (SE), south west (SW) and north west (NW).

To give a direction, start at the place you are going from. Then say which way you must go to reach the place you are going to. If a school is up and to the right of a station on the map, the school is north east of the station. The direction from the school back to the station is the opposite, south west.

Symbols and numerical information

An OS map uses small symbols instead of words. There is a key on the map and you are expected to use it. Symbols show basic features such as a church with a tower or spire, a public house (PH), a post office (P), a railway station, a campsite, a picnic site, a viewpoint and a car park. Learn to read the key quickly and match the symbol to the feature.

The map also gives you numbers. These are called numerical information. You can use them as evidence in answers.

• Counting: the number of buildings, car parks or campsites in a square.
• Area: each grid square is 1 km². A lake that covers about three squares is about 3 km².
• Distance: measured as shown above, for example the length of a footpath or a road.
• Grid references: to name the exact position of a feature.

In a longer answer, use at least one grid reference or measurement. Writing "there are four campsites in squares 3471 and 3472" is stronger than "there are some campsites".

Exam-style question

Look at an OS map extract at a scale of 1:50 000. A viewpoint is in grid square 4567. A village hall is in the next square to the east.

(a) Give the four figure grid reference of the square with the village hall. [1 mark]

(b) The viewpoint is 6 tenths of the way across its square and 3 tenths of the way up. Give the six figure grid reference of the viewpoint. [2 marks]

(c) The footpath from the viewpoint to the village hall measures 9 cm along a piece of string. Calculate the real distance in km. [2 marks]

(d) The campsite is north west of the village hall. State the direction of the village hall from the campsite. [1 mark]