The records in this series relate to the retriangulation of Great Britain begun in 1935, and constantly updated since, until finally superseded in the late 1990s.

The series contains the triangulation station files. These are a working record for each station, containing a site map and sketch for ease of location, the history of the station (when planned and built, with details of events), and observing details. The Ordnance Survey Act of 1841 gave Ordnance Survey's surveyors a legal right to enter into and upon any land for survey purposes. Details of the Landowner are therefore also held in the files. Files may also contain photographic images.