Ordnance Survey (website, Wikipedia) is the national mapping agency for Great Britain (the United Kingdom excluding Northern Ireland). Although other organisations have mapped Great Britain, the Ordnance Survey has the most detailed maps, and many organisations license the O.S. data products for use elsewhere. This includes Google Maps coverage of the UK for example (licensed via Teleatlas).
There is a great deal of national pride in Ordnance Survey, which has been producing beautiful maps for over a century. The landranger series of walking maps are particularly popular. Originally this was a government (crown) organisation funded and owned by the taxpayer, these days it operates a "trading fund" which must fund itself like a profit-making business. Hence the license …
In the past the maps have all been covered by "crown copyright", and the ordnance survey set a precedent for enforcing extremely strict interpretation of "derived works". This coupled with very high license fees was a huge cause of frustration for many people. In fact it was this which gave rise to the OpenStreetMap project! (see History of OpenStreetMap)
Main page: Ordnance Survey Opendata
Ordnance Survey became a central target of open data campaigning in recent years. This pressure, and the pressure of emerging competition exerted by OpenStreetMap (as well as other neo-geo technologies such as Google Maps) has forced the government to hold an Ordnance Survey Consultation in 2010 and to open up some of their data. We helped make this happen!
Some old OS maps have fallen out of copyright. As such there are absolutely no restrictions / attribution requirements. They are all raster maps (scanned images) which we can trace.
- New Popular Edition - 1in to the mile (1:63,360), 1940's maps used as an early data source for OpenStreetMap.
- 7th Series - 1in to the mile (1:63,360), 1950/60s. Better detail level. Coming out of copyright. Some areas available
- Provisional/First Edition - 2027 map sheets, printed from the 1940's right through to the 1980's, at 1:25,000.
Triangulation stations aka 'trig points'
Ordnance Survey has kindly agreed to publish its archive of triangulation station data under the Open Government Licence v3, allowing usage in OSM. User:Gregrs has written an analysis of this data in relation to OpenStreetMap.
See https://en.wikipedia.org/wiki/Triangulation_station#United_Kingdom for further information about what trig points are and what they are used for.