Benchmark hunting is something I’ve been doing since 2018. They are markings carved into stone by Ordnance Surveyors in the 19th and 20th century when measuring height above sea level. The benchmarks or crowsfeet were left there for surveyors coming years later as a reference point. They were also marked on Ordnance Survey maps with a similar symbol. Some people like me go around and look where they can still find them. And some of those people then add them to OpenStreetMap, from where they can be extracted to make maps like the one below.
I use the following parameters when tagging them on OSM (not all of them all the time)
man_made=survey_point benchmark=yes source=survey [plus the historical maps they are shown on] survey_date= width= [ideally in cms] height= direction= [in degrees, but I started that late, so very few have that value] ele:1913= [elevation on 25'' Ordnance Survey map in feet) mapillary= descriptio= [for cases when the stones bearing them were moved or other oddities]
On my umap, you can actually see those values by clicking on the grey dots: