Cartographic skills - EdexcelHow to read Ordnance Survey maps

The Ordnance Survey (OS) is the mapping agency for Great Britain. It creates up-to-date paper and digital maps for individuals and businesses to use.

Using a key

OS maps show physical and human features as symbols. This makes the maps easier to read. Each OS map has a key to show what the symbols mean.

Scale and distance

Maps show objects as being much smaller than they are in real life. The relationship between the features on the map to the real size on the ground is called the scale. Scale is shown as a ratio, eg 1:25,000 means that 1 cm on a map represents 25,000 cm or 250 m in real life.

OS maps come in different scales:

• Large-scale maps - 1:1,250, 1:2,500 and 1:10,000. Features appear larger on the map. This may be used for individual towns and cities.
• Small-scale - 1:25,000, 1:50,000 and 1:100,000. Features appear smaller on the map. This may be used to show larger areas or regions.

Four and six-figure grid references

Grid references accurately locate places on a map. Every OS map has a grid, which is shown using faint blue lines. The lines across the bottom of the map are called eastings as they travel towards the east. The lines up the side of the map are called northings as they travel towards the north.

Four-figure grid references locate a place or object within a grid square. Four-figure grid references are found as follows:

1. First, write the eastings number of the bottom left corner of the square (eastings are found along the bottom of the map). The number will have two digits eg 13.
2. Then, write the northings number of the bottom left corner of the square (northings are found along the side of the map). The number will consist of another two digits giving a total of four, hence the name four figure grid reference.

Six-figure grid references locate a place or object within a specific part of a grid square. Six-figure grid references are found as follows:

1. Write the two figure eastings number, but then add a third number to show how many tenths of the way across the grid square the place or object lies.
2. Write the two figure northings number, but then add a third number to show how many tenths of the way up the grid square the place or object lies.

Contours and spot heights

On a map, height is shown in metres above sea level. Spot heights show the height of a particular point on the map.

Contour lines are added to a map to show height and gradient. On OS maps they are shown as thin orange or brown lines, some of which have the land height written on them. The lines join areas of equal height:

• Contour lines that are close together show land that increases or decreases in height quickly. This is steep land.
• Contour lines that are far apart show land that increases or decreases in height slowly. This land is gently sloping.

We can use contour lines to create cross-section diagrams of landscapes. This helps to match contour lines on the map to landscape features in real life, eg hills, valleys and spurs of land.

Site and situation from an OS map

It is also important to be able to interpret the site and situation of a settlement from an OS map.

The site refers to the local and regional characteristics that encouraged the initial siting of the settlement and the situation is the site's location in relation to nearby physical and human features.

There are both physical and human factors that affect the site and situation of settlements which can be interpreted from an OS map.

Site

• Water supply - look for rivers coloured blue on the map.
• Fertile land - look for farms with the symbol FM.
• Wet/dry point - look for raised land above the river often through spot heights or contours.
• Fuel supply - look for a local forest.
• Building materials - look for a quarry.
• Defensive site - look for the word castle.

Situation

• Gap town - look for a valley in a chain of hills using the contour lines.
• Communications route - look for a crossroads or a railway junction.
• Bridging point - look for a bridge across the river.