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The Ordnance Survey (OS) is the mapping agency for Great Britain. They create 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.

A graphic scale is depicted using a line, with separations marked by smaller intersecting lines, similar to a ruler.

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 and 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 and 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.

Cross-sections of river depths are drawn using negative numbers so that the line graph looks like depth, rather than height.