concrete pillar is a Triangulation Pillar or Triangulation Station,
sometimes abbreviated by map-users to trigpoints. These were used in
the making of maps as the Ordnance Survey sited their instrument (a
theodolite) on top of the pillar and were able to take accurate bearings
to other Triangulation Stations. As bearings between at least three
points allowed places to be mapped, this process was known as 'triangulation'.
Trigpoints are normally shown on OS maps as blue triangles with a dot
in the middle - a replica of the brass plate located on the top of the
concrete pillars. Trigpoints are so popular with some people they have
spawned their own website www.trigpointinguk.com!