@Cameleopard Not from the recent examples I can grub up with an image search. I may have missed it, tho. They're kind of internal to the process of surveying without GPS.

OS still(?) use a blue upward-facing equilateral triangle for the trig points, but I think that's the extent of their mapping their own reference points. I think these marks occur in other places on the ground too.

Perhaps historical maps might have it, or a more recent book on the subject

@Pigsonthewing @openstreetmap