On 12/06/2021 19:16, Nick Odell wrote:
On Sat, 12 Jun 2021 18:47:43 +0100, Roger Mills
wrote:

On 12/06/2021 16:09, N_Cook wrote:
Needed for accurate , ie order of 1cm absolute levels determination.
The benchmark of consideration is just one brick remaining in-situ after
vehicle damage to a wall years ago.
Reconstructing the mark from the remaining one 2 parts of the trident
mark, taking a tracing off nearby ones, places the horizontal level mark
about 2 cm above the centre of the replacement stretcher above.
Looking around at other banchmarks in different towns and cities all
have the level cut mark at the exact centre of stretchers.
Anyone know what the criteria was, ie always without exception cut to
the centre of a brick and then surveyed to that point, X,Y and Z?
While at it ,as wwwland is no help. For flush bracket later forms is the
level mark the centre of the shortened upper recess hole , in the
styalised "blocky" form of the old War Dept trident arrow cut marks?



If I have correctly understood what it says at
https://www.ordnancesurvey.co.uk/bus...et/legacy-data
you ain't going to get an absolute accuracy of anything like 1cm anyway.

The OS Blog
https://www.ordnancesurvey.co.uk/newsroom/blog/25-years-since-last-benchmark
suggests that thanks to subsidence and other geological events, the
old benchmarks can't be relied on for really accurate measurements.

The one on the front wall of the house next-door-but-one has the line
about 2cm down from the top surface of the stone it is carved into.

Nick


Yes one mark near me is useless as its on what is now a leaning wall.
Another one I knew it was on the maps but had not seen until I was on a
bus and the sun reflected off the mark. Carved ? cut mark into
acid-etched blue granite aggregate concrete of a bridge pillar, so only
some short segments of the 4 lines are sort of visible around the bits
of granite.

--
Global sea level rise to 2100 from curve-fitted existing altimetry data
http://diverse.4mg.com/slr.htm