On 12/06/2021 19:26, N_Cook 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.


Fair enough for the optical-surveying sight lines of 1km or so.
For this use with auto-level or laser-level with single-leg survey, over
less than 100m, ie 50m sighting arms , 1cm is possible.
OS benchmarks quoted on "National Grid" series maps to 1cm , Z-axis.
As part of calibration routines my local group of benchmarks , checked
both ways, are still within 2cm of the 1970 levels, over a total
multi-leg span of about 400m. Using this fragment of benchmark as its
the closest, so avoiding mult-leg surveying to the next nearest and the
inherent inaccuracies, hence the query here as to validity of using of a
partial mark.
At least you did not say use d-GPS, nowhere near 1cm is possible there.
The local archaelogists use their "total stations" for on-site surveying
, but for the site reference points they cough up 20 quid a pop to OS
for the details of the nearest old OS benchmark and optically they
survey to that,Â* having been seriously mis-placed by d-GPS for national
placement


Interestingly, the field next to my house has recently become a building
site on which nearly 150 houses will be built. The developers have been
using devices similar to
https://leica-geosystems.com/en-gb/p...s/leica-gs18-t
to mark out all the plots *and* to determine the levels for the
foundations and all the drains. They claim that they can measure
absolute altitude to an accuracy of a few mm. Seems magic to me because
normal GPS equipment has a tolerance of tens of metres on altitude.
--
Cheers,
Roger