They are Ordnance Survey "bench marks" which are precisely surveyed
their height above the zero level for the contours seen on ordnance
maps. Their height is measured using precise methods and is accurate
about a cetimetre.
Like the "Trigonometrical Points" that you see on the tops of many
which are accurately surveyed in plan position (latitude and
and/or grid reference), they are largely being supplanted by Global
Positioning Systems (GPS) that use the same orbiting satellites as
systems in some expensive cars - and motorhomes!
Hope this helps!
Just to add that Ordance surve maps will generally give a mark and a
spot height value for these marks. So if need to find one, or have
found one, the height is known.
If you need the height verified, you can ask OS for the most recent
reading. Some Bench marks are used regularly where new devlopment is
taking place and there heights amended as the country side gently
moves up and down ( as it does )
TonyIsh UK (The other Tony)