TYPES OF TRIGPOINT
The classic trigpoint and a welcome sight for hill walkers everywhere! A concrete pillar about 4' high, with a flush bracket near the base and a circular plate on top with groves for mounting a theodolite. See the main Pillar article for further details of triangulation pillars.
FBM - Fundamental Benchmark
An underground chamber topped with a short granite pillar. There are approximately 207 FBMs across the country. See the main FBM article for further details of FBMs.
Other survey marks
As well as the familiar pillars, the Ordnance Survey also installed a large number of smaller survey stations. There are many different varieties of these, including Surface Blocks, Buried Blocks, Rivets, Bolts, Fenomarks, Berntsens and Curry Stools. Like the pillars, these stations were used both for the original 1936 retriangulation, and some also form part of the network of Passive Stations.
The 1936 retriangulation used not only installed survey marks, such as the above, but also made use of existing features, such as flagstaffs, towers and the like. In many cases, these Intersected Stations are associated with a smaller, fixed survey mark. For example, a (more permanent) bolt may be installed near to a (more visible) flagstaff.
There are 163 active stations in the T:UK database, (including those which are no longer used), which are sited at about 30 locations around the country. These are GPS receivers which constantly log their readings (known as Rinex data) on the Ordnance Survey website. This allows a very high level of positional accuracy to be achieved, without the need to physically occupy one of the Passive Stations.