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A 'Centre' is common type of triangulation mark, informally defined as the horizontal mid-point of any prominent feature of a building. There are 1756 instances of centres in the OSGB36 list [1]. Of these, 1412 are towers (most often churches, also water towers, mills, aero, castles, etc). Other Centres include tanks, colliery headgear, windmills and monuments.

A Centre NZ42/T62

The Centre is not occupiable; i.e. observations can be made to, but not from, the mark. Originally a “Centre” was used primarily to avoid the need to beacon a station [2]. However, the Centre may be one of several marks comprising a trig station, for example bolts or rivets, which then allow observations to be made from it.

Unless the Centre is part of a station which also includes an occupiable mark (bolt, rivet) then the station name is designated in the OSGB36 list as an Intersected Station.


  1. https://www.ordnancesurvey.co.uk/legacy/docs/gps/CompleteTrigArchive.zip
  2. Email from Ian Wilson to the Trigonomy mailing list, 10/06/2012