In the 1700’s the first baseline for triangulation was measure on Hounslow Heath. It first used wooden then iron and finally glass rods for better accuracy to measure the baseline of the first triangle. Subsequent triangulations formed new baselines, then in 1936 a whole new system of triangulation was developed by Ordnance Survey along with a whole new set of markers on the ground for the triangulation points.
The last triangulation of Britain took place between 1935 and 1962 with the now familiar triangulation pillar being used for the first time. Along with the pillars there were several other types of markers employed for the survey points.
Buildings were often used, especially ones which would not probably be pulled down in the future, churches were a favourite, Bakewell was one, as was the Hunting Tower and its flagstaff on the Chatsworth estate.
Fundamental Benchmarks were the cornerstone of the whole system, these were protected by iron fencing. The FBM as they were known were seated on bedrock and were used as definitive levelling points as the one on the Mortimer Road on Broomhead Moor.
Surface Blocks like the one at Hollingworth Head, there is also one near Big Moor formed part of the triangulation measurements. They lie close to the surface but are hard to find as they are often covered with grass. Even harder to find are Surface Bolts such as the Laddow Rocks one. It is amazing that a small bolt has survived all this time.
And sometimes a triangulation point just isn’t one. This is the case of the Hey Edge Pillar which is marked on the map as a Pillar, but does not have the blue triangle on the map. That is because whilst it was built it was never used for triangulation. It was used for levelling, but as I have said that is a totally different set of measurements. Hence it does not qualify as a trig point to Ordnance Survey.
Nowadays with GPS triangulation by hand is no longer required and the Ordnance Survey network of triangulation points is falling in to disuse, apart from a few that is. The Triangulation Pillar at Harland South forms part of the Global Positioning System network, as does the Fundamental Benchmark on Broomhead Moor. So even though GPS is now the dominant measuring system for position, it still relies on spots on the ground for accuracy.