What is a Trigpoint?
It is usually a hand-cast concrete pillar, 4 feet high and 2 feet square at the base, tapering towards a flat top.
It is part of a network of surveying stations that were used by Ordance Survey for mapping. Each pillar is in view of several others
so that their position may be triangulated.
Hence the formal term is triagulation pillar, but this is usually shortened to trig pillar or trig point. On this site this is abreviated even further to TP.
TPs usually have a 'spider' or 'top plate' used to fix a theodolite or other ordnance surveying device to. Many have a Flush Bracket fixed to the side that has an identifying number on it. There are different designs, as you will discover as you read condition reports and see the photographs that these contain.
Below left is a spider and at the right, one type of flush bracket. It is attached to the Triangulation Pillar at Carncormack located at D 1691 1433.