N_Cook used his keyboard to write :
With this type of benchmark
https://upload.wikimedia.org/wikiped...cket_G4863.jpg

Is the level,recorded on maps, given by the flat ledge immediately above the
upward pointing broad arrow, or half way up the recess involving that ledge
or some proprietary position, only known within OS, relative to the 5
location recesses in the plate

The horizontal line is the datum.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk