Geograph - photograph every grid square

The process of placing trig points on top of prominent hills and mountains began in 1935 to assist in the accurate retriangulation of Great Britain. The Ordnance Survey's first trig point was erected on 18 April 1936 near Cold Ashby, Northamptonshire. In low-lying or flat areas some trig points are only a 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.