Hello
I have a ground level calculated relative to Newlyn datum.
This level is in Perth, Scotland, and I need to convert it to the local datum for Perth. Can I find the conversion required for Perth relative to Newlyn anywhere online? No!
Any help greatly appreciated!
Can you relate the Newlyn datum to the nearest local OS benchmark (And let the OS do the calculation for you)? The benchmarks are usually correct to a stupid degree of accuracy (I can remember having to re-level one in year ago and believe it involved walking getting on for 18 miles by the time we’d checked it back to the neighbouring benchmarks to the satisfaction of the man with the calculator (it was that long ago)).
Aren't all OS Benchmarks calculated to Newlyn Datum?
I know that the BM network is not fully maintained and loads are missing or inaccurate but there are still about 150 primary BM's around the country which are. (FBM's?)
If you're looking to find a level near you in Perth then this might help. (turn off the heat demand layer first)
its mainland UK? so should already be tied to newlyn?
Richie - I got the level from OS maps online - they are all tied back to the Newlyn datum. OS maps does give the details for the nearest benchmark, but all of the details are relative to Newlyn only!
Jam - the local water authority will only accept levels to Perth datum.
ask them what the transformation is then.
Jam - I did. They said I had to do my own calculations.
If you mean relative to the Perth FBM then that's 136.353 AOD
As far as I know Newlyn is the datum for all terrestrial survey work in the UK. The level of the Perth FBM that Percy has linked is using the Newlyn datum. Marine charts have a local datum, usually based on the level of low tide, and river level monitoring stations each have their own datum. Individual construction sites sometimes have their own datum, partly for convenience and partly because it's often more important that the levels are consistent within the building than to anything external and the whole site can settle as the additional mass is built up.
All the replies above are basically bang-on - published OS height datum is all given relative to Newlyn, although it was originally (late 19th century) calculated to a Liverpool datum. That was often shown on maps in feet and inches though, rather than metres above ODN.
If you can start from a more local benchmark to you than the Fundamental out near Deuchny Wood, then this site OS Benchmark Locator will show you all the recorded BMs in a given grid square; it should be fairly easy to track down the nearest one to your site
the local water authority will only accept levels to Perth datum
Not sure what you’re doing but from experience where the water company thinks their pipes are and where they actually are are not always closely related (Sorry I’m basing this on Severn Trent, the epitome of modern management theory and efficiency).
Its usually worth making sure that whatever survey you are working to records the actual cover and invert levels of any inspection/access/valve chambers nearby so they relate directly to your measurements and should the need arise would allow you to adjust all your levels to where they think they are.
I’ve not worked with Scottish after authorities but with the English ones I’ve worked with the bureaucracy is so clunky that its is often best to fit in with them rather than argue the point. The last connection I was involved with related to trying to find a pipe which turned out to be on the other side of the road and 1.5m deeper than their records said it was.
go and find a construction site, have a chat (with a couple of beers) with the engineer, who will have a GPS accurate to sub 10mm vertically.
