Ordnance Survey Maps: Grid References & Scale

Ordnance Survey (OS) maps are the most detailed and widely-used maps of the United Kingdom. At GCSE you are expected to read and interpret OS maps with confidence, using grid references to locate features precisely. This lesson covers the grid reference system, map scale calculations, and how to extract information from OS maps.

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What Is an Ordnance Survey Map?

The Ordnance Survey is the UK's national mapping agency. OS maps cover the whole of Great Britain at various scales and are used by walkers, planners, geographers and emergency services.

There are two main series used at GCSE:

Series	Scale	1 cm =	Grid square size	Best for
Explorer	1:25,000	250 m	1 km x 1 km	Detailed fieldwork, walking
Landranger	1:50,000	500 m	1 km x 1 km	General-purpose, exam extracts

On both series, the blue grid lines divide the map into 1 km squares.

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Grid References

Grid references allow you to pinpoint locations on an OS map. There are two types:

Four-Figure Grid References

A four-figure grid reference identifies a 1 km square. To give one:

1. Read along the bottom of the map (the eastings) — find the vertical line to the left of the square
2. Read up the side of the map (the northings) — find the horizontal line at the bottom of the square
3. Write the easting first, then the northing

Example: The square with easting 32 and northing 45 is written as 3245.

Exam Tip: Remember: along the corridor and up the stairs — eastings first, then northings.

Six-Figure Grid References

A six-figure grid reference pinpoints a location to within 100 m by dividing each 1 km square into tenths.

1. Find the easting of the left edge of the square (e.g. 32)
2. Estimate how many tenths across the feature is (e.g. 7 tenths = 327)
3. Find the northing of the bottom edge of the square (e.g. 45)
4. Estimate how many tenths up the feature is (e.g. 3 tenths = 453)
5. The six-figure reference is 327453

Reference Type	Digits	Accuracy	Example
Four-figure	4	1 km²	3245
Six-figure	6	100 m²	327453

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Practising Grid References

Here is a worked example. Imagine a map extract with the following grid lines:

• Eastings: 50, 51, 52, 53, 54
• Northings: 20, 21, 22, 23, 24

A church with a tower is located roughly in the middle of the square between eastings 51–52 and northings 22–23.

• Four-figure reference: 5122 (left easting, bottom northing)
• Six-figure reference: 515225 (5 tenths across, 5 tenths up within the square)

Exam Tip: Practise six-figure grid references until they are second nature. Many students lose marks by reversing eastings and northings. Always go along before you go up.

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Measuring Distances on OS Maps

Straight-Line Distance

1. Use a ruler to measure the distance between two points in centimetres
2. Convert using the scale:
   • At 1:25,000: multiply cm by 250 to get metres, or by 0.25 for km
   • At 1:50,000: multiply cm by 500 to get metres, or by 0.5 for km

Example: Two points are 6.4 cm apart on a 1:50,000 map.

• 6.4 × 500 = 3,200 m = 3.2 km

Curved/Winding Distance

For a winding route (e.g. a river or road):

1. Place a piece of string along the route, following the curves
2. Straighten the string against a ruler
3. Convert using the map scale

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Calculating Area

You may be asked to calculate the area covered by a feature:

1. Count the number of full grid squares covered
2. Estimate partial squares (count any square more than half-covered as a full square)
3. Each grid square = 1 km × 1 km = 1 km² on both 1:25,000 and 1:50,000 maps

Example: A woodland covers 4 full squares and 3 partial squares (2 of which are more than half-covered).

• Total = 4 + 2 = 6 km² (approximately)

Exam Tip: Always state the unit (km²) and say "approximately" since you are estimating partial squares.

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Relief on OS Maps

Relief means the shape and height of the land. OS maps show this using:

Contour Lines

• Brown/orange lines connecting points of equal height above sea level
• The contour interval (height difference between adjacent lines) is usually 10 m on 1:25,000 maps and 10 m on 1:50,000 maps
• Where contour lines are close together, the land is steep
• Where contour lines are far apart, the land is gentle or flat

Spot Heights and Triangulation Pillars

Feature	Symbol	What It Shows
Spot height	A dot with a number	Exact height in metres at that point
Triangulation pillar	A small triangle	Exact height; used as a survey reference

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Describing the Physical Landscape from an OS Map

When asked to describe relief, use the following approach:

1. Height — quote specific figures from contour lines or spot heights (e.g. "the land rises from 50 m in the valley to 320 m at the summit")
2. Steepness — refer to contour spacing (e.g. "closely-spaced contours on the northern slope indicate steep land")
3. Shape — identify features such as:
   • V-shaped valley — contour lines form a V pointing upstream
   • Spur — contour lines form a V pointing downstream (land jutting out)
   • Flat land — widely-spaced or absent contour lines
   • Ridge — elongated area of high land
   • Saddle/col — a low point between two peaks
4. Drainage — note rivers, streams, lakes and their relationship to the relief

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Describing Human Features from an OS Map

You should be able to identify and comment on:

Feature	What to Look For
Settlement pattern	Nucleated (clustered), dispersed (spread out), linear (along a road/river)
Settlement size	Number of buildings, area covered
Transport	Roads (A-roads in red, B-roads in orange, motorways in blue), railways, footpaths
Land use	Farmland, woodland, urban areas, industrial zones
Tourism/leisure	Campsites, visitor centres, nature reserves, beaches
Services	Schools (Sch), churches (+), post offices (PO)

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Linking Physical and Human Features