What & Why

[kenkew]

This is. or rather this was a popular chiselled mark. What is it and why was it made?
(Not very clear here, but looks sort of like this...
   _
  /|\

[kenkew]

[Robert_Brenchley]

A surveyor's mark.

http://en.wikipedia.org/wiki/Benchmark_%28surveying%29

[kenkew]

Well done Robert. 10/10... I'm hoping you had the answer before checking the pic name...?
(Used to mark height above sea level...these days we use other means...sad me thinks.)