With this type of benchmark
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
N_Cook used his keyboard to write :
The horizontal line is the datum.
Yes but by todays standards, they're not that accurate. ISTR all the physical datum lines and trig. point have been abandoned by OS some time ago. They even depend on private individuals to maintain them. I used to maintain a trig point myself.
The traditional chisselled benchmark, the level was the centreline of the horizontal chissel mark, ie most recessed, rather than the lower edge, equating to the horizontal ledge of the flush bracket type.
I thought dGPS/total station 2 station GPS had taken over but I was wrong, according to the head of local archaeological unit. They've had so much error with dGPS , that they now stump up 20 GBP a pop for a convenient OS reference point, X,Y,Z data close to any dig they now start. Never using dGPS for any basepoint determination of a site, later on site yes, but not the initial absolute base reference X,Y,Z. They could not even trust dGPS X and Y fixes as the worst error on that, placed one of their sites in a neighbouring building, it emerged later on.
Very conveniently, we have one just 100 yards from here.
I was doing some laser levelling from a flush bracket benchmark, yesterday. With beeper 1200m range the specs say, but +/-0.5mm per 10m not so good despite an espensive model, I wonder what dumpy level specs are
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