Benchmark technicality (Ordnance Survey ones that is)

[N_Cook]

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?

[Roger Mills]

If I have correctly understood what it says at

you ain't going to get an absolute accuracy of anything like 1cm anyway.

[Nick Odell]

The OS Blog

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

[N_Cook]

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, having been seriously mis-placed by d-GPS for national placement

[N_Cook]

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.

[Roger Mills]

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

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.

[N_Cook]

I would not trust the American military as far as I could see them using high power binos. These days d-GPS tends to use a base station connected by cell-phone network to the surveyors station. The base station is fixed in position and then your outlier is deemed fixed to the base station receiving the same satellite signals and timing comparison and adjustment also deemed fixed for each usage. Forgetting about the about 50 types of routine corrections to any sort of GPS reading , from oxygen content and ionisation levels in the upper air , to relativistic effects of fast space craft, the local effects of the likes of trees and steelwork gives an incalculable to mess things up and cm accuracy is impossible, especially in the z sense. This is how the local archaeologists went wrong, previously using d-GPS, they had an important site that was not only entirely wrong in the z sense but was placed x,y sense in someone elses grounds. Since then they've coughed up 20 quid a time to OS for traditional optical surveying into the national grid structure.

[williamwright]

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

Bill

[N_Cook]

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.