A map scale correctly reduces the actual distance on the ground to a corresponding distance on a map. Printing the scale on the map means the reader can work out the accurate distance between two points.
There is a standard ratio format for map scales e.g. “1: 25,000”. The first number is the unit on the map and the second number is the distance in real life of the same unit so 1: 25,000 means that 1cm on the map corresponds to 25,000 cm on the ground.
A distance of 25,000 cm is hard to visualise. Often a map scale will have an additional written scale giving a map unit and a real life unit – i.e. it converts one unit of measurement to another. For example, a UK Ordinance Survey (OS) map reads: “1:25,000 scale, 4cm to 1km”. The first scale corresponds to 25,000 cm on the ground as before. But the second ‘written scale’ of ‘4 cm to 1 km’ makes it easier to convert as 4 cm on the map represents 1km on the ground. Our Huge Geological Map of the World is a 1:22,000,000 scale. This means that 2.5 cm is equal to 500 km.
Putting a large area on a map, e.g. a world map, requires a small scale - this converts a large area into a small map. Our original map, the Future Map, is a 1:40,000,000 scale. The area shown here is around 15cm, so this image shows approximately 3,750km.
Putting a smaller area on a map, e.g. a street area of London, requires a larger scale to show more detail. Our London City Map is a 1:30,000 scale, meaning the 2.5km journey from Greenwich Park to the O2 fits into the same 15cm area.
Most maps include a graphic scale bar, which has two functions – to enable the reader to easily visualise distances on the map; and when enlarging or shrinking the map, the graphic scale remains true.