Cartographic Skills – OS Maps
Find out about contours, scale, keys and grid references when using OS Maps.
The Ordnance Survey (OS) is the mapping agency for Great Britain. It is responsible for producing detailed digital and paper maps for businesses and individuals to use.
OS maps show human and physical features using a range of different symbols. A key is used to show what the different symbols mean. The Ordnance Survey have shared a key for a 1:25 000 and 1:50 000 OS map.
North is always at the top of an ordnance survey map. To find out more about directions take a look at our dedicated page on compass directions
Maps show features as being much smaller in size than they are in real life. The link between the features on a map and their real size on the ground is called the scale. Scale is shown as a ratio. For example, if a map has a scale of 1:50,000 it means that 1 cm on a map represents 50,000 cm or 500m in real life.
If a map has a scale of 1:25 000 it means 1 cm on the map represents 25 000 cm or 250m in real life.
Ordnance survey maps come in a range of scales. Large-scale maps such as 1:1,250, 1:2,500 and 1:10,000 usually show individual towns and cities. Smale-scale maps such as 1:25,000, 1:50,000 and 1:100,000 show features smaller and often show larger areas or regions.
There is a very useful calculator for working out distances on a map on the MapTools website.
Grid references allow people to accurately locate places on a map. Every OS map has a grid of faint blue lines printed on it. The lines that run from top to bottom are called eastings. Those that run horizontally from left to right are called northings. To find out more about grid references take a look at our dedicated page about grid references
To find out more about 6 figure grid references take a look at our dedicated page about grid references
Height is represented on OS maps through contour lines, spot heights and triangulation pillars.
A spot height shows the height of a specific point on a map. It is represented as a black dot with the height of the point written in black.
Trig points or Triangulation Pillars are another way of spotting the top of a mountain on a map. The symbol for a trig point on an OS map is a small blue triangle with a dot in the middle. They are real concrete pillars that are placed at particular places which are usually the tops of hills or mountains. The height of the triangulation pillar is written next to it.
Contour lines are light brown lines that join up land that is the same height. Height is shown in metres above sea level. Some lines have the height of the land written on them. Using contours you can work out the height and gradient of the land. Contour lines that are close together show that the height of the land increases or decreases quickly. This is steep land. Where contour lines are further apart the height of the land increases or decreases slowly. This represents gently sloping land.
Use the images below to explore related GeoTopics.