The concrete pillar about 1.2m (4ft) tall located on the rim of the Hillfort’s ditch at the highest point on Hunsbury Hill and, indeed, Northampton, is a Trig Point.

There is probably twice as much concrete below the surface as above. There is a brass plate set into the top to enable a surveyor’s theodolite to be fixed to it and an Ordnance Survey Bench Mark (OSBM) on the side to provide a known elevation marker. The theodolite would have been able to see at least another two Trig Points, all on high points.

It formed part of a network of 6,500 similar points throughout the UK, constructed in 1935-1936, to enable a re-triangulation of the UK to take place. The first of the 6,500 Trig Points was built in Northamptonshire, at Cold Ashby, about 15 miles north-west of Northampton. ‘Trig’ is derived from the word ‘trigonometry’, on which the triangulation is based. Trig Points are shown as blue triangles on OS maps and are now mostly redundant as OS now uses satellites and sophisticated GPS.

The original or principle triangulation was carried out 1783-1851. It was started a few years before the start of the Napoleonic Wars, so as to provide accurate maps of the south of England in order to prepare defences in case of invasion by Bonaparte. It might not be coincidence that the re-triangulation was commenced a few years before the start of World War II and, like the principle triangulation ,was not completed until after the war.

It is no coincidence that our Trig Point is on the rim of the Hillfort’s defences, as the top of Hunsbury Hill is not only the highest point in Northampton, but it has a 360° viewshed, meaning that when chosen to be the location for an Iron Age Hillfort, it was easy to defend because no-one could get within at least 5km of it without being seen.

This is the eighth in the Friends of West Hunsbury Parks series of monthly Fun Facts about the local area. You can find the others on our website and Facebook. Thank  you for your comments on the previous Fun Facts. Please keep them coming if you find this one interesting.