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.
However, this simple concept of map scales is complicated by the curvature of the Earth's surface, which forces scale to vary across a map. We explain Map Projections in our posts Part 1 and Part 2.
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.