few metres above sea level and one is even at −1 m (near Little Ouse, Cambridgeshire, TL61718 89787). When all the trig points were in place, it was possible in clear weather to see at least two other trig points from any one trig point, but subsequent vegetation growth means that this is not necessarily still the case. Careful measurements of the angles between the lines-of-sight of the other trig points then allowed the construction of a system of triangles which could then be referenced back to a single baseline to construct a highly accurate measurement system that covered the entire country. In most of the UK, trig points are truncated square concrete (occasionally stone) pyramids or obelisks tapering towards the top. On the top a brass plate with three arms and a central depression is fixed: it is used to mount and centre a theodolite used to take angular measurements to neighbouring trig points. A benchmark is usually set on the side, marked with the letters "O S B M" (Ordnance Survey Bench Mark) and the reference number of the trig point. Within and below the visible trig point, there are concealed reference marks whose National Grid References are precisely known. The standard trig point design is credited to Brigadier Martin Hotine (1898–1968), head of the Trigonometrical and Levelling Division of the Ordnance Survey. Many of them are now disappearing from the countryside as their function has largely been superseded by aerial photography and digital mapping using lasers and GPS. To quote from a page at the OS site: "Like an iceberg, there is more of trig pillar below the surface than above it." From the same source: "Today the receivers that make up the OS Net network are coordinated to an accuracy of just 3 mm over the entire length of Great Britain." See also Struve Geodetic Arc Bilby tower Boundary marker Geodesy Ordnance Survey Retriangulation of Great Britain Survey marker Trigpointing is a pastime in which people individually go out, find and log the location of trig points. Cover of Martyn Bennett's album Grit References External links TrigpointingUK GPS Waypoints of all UK Trigpoints Photos of examples of trig points in the UK on geograph.org.uk Database of trig points in the United Kingdom Repères de nivellement. French I.G.N Serveur de fiches (maps). French I.G.N Database of trig points in New Zealand "Trig pillars we salute you", blog