What is ordinance survey map?

During the English civil war Oliver Cromwell, leader of the Parliamentarian side discovered that he had great difficulty hitting anything with his artillery.

He realised that the root of the problem was the very poor quality of the available maps of England. When his side won the war and he became a dictator he set up a government controlled and funded organisation to carry out a proper survey of the country. Because it was intended for use by artillery, it was called the Ordnance Survey.

The English decided that one civil war was more than enough, so the artillery never needed these maps, but all sorts of other people did. The O.S. still exists and keeps the maps up to date. Their maps are widely available and very popular.