On 15/06/2021 01:08, williamwright wrote:
On 12/06/2021 18:47, 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.

They haven't used benchmarks for donkey's years anyway.

Bill

Ah, over-reliance on hi-tec.
The last couple of days , back to normal now, major port Southampton VTS
(Vessel Traffic Services) had been outputting the bubbler tide-gauge
levels as being 0.3m too high for 2 days (think "Ever Given"). No one
could be bothered looking out a window to a tide pole for mark1 eyeballs
to check if there there was a major problem with the hi-tec tide gauge
output. Then pull the plug on the dangerous hi-tec output, no output is
safer than erroneously high output.

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