Have you bagged any trigs? Where? For the uninitiated, I’m not taking nonsense! You might have occasionally noticed a structure like the one in the photo, usually with a number on the side and curious symbol on the top. There are over 6000 of them in the UK. Some walkers pride themselves on 'trig-bagging', sharing their finds online.
These mysterious small structures are called Trig Points, or more correctly, "triangulation pillars". They're usually on the highest point in the area and theoretically with a direct line of sight to the next. Angles between pairs of trig points can be calculated (or triangulated) with a theodolite. They were first created in 1936 when a project started to map the shape of Britain. I think their development eventually led to the creation of OS maps.