Just in case anyone is interested in what happens when the folks in the RHB group survey/resurvey hills to check/change elevation data for their lists (in the UK), you might be interested in the following:
snip
Yesterday Graham Jackson, Myrddyn Phillips and I in conjunction with BBC CountryFile and the OS surveyed Milk Hill and Tan Hill. We had already spent a day prior to this filming sorting out the highest point of each hill and taking our own GPS readings so we knew the result was not exactly as the latest 1:25000 OS maps suggest! The broadcast goes out at 7pm on BBC1 on Sunday 23rd August and will be the first piece on the programme. So if you want to know the highest point in Wiltshire you need to watch the programme.
Yes, thanks Marc. Like Phil, I normally watch it now it’s on a Sunday evening but I’ll set an alarm / warn everyone in the househld etc just to be sure.
So Milk hill is measured as the highest point in Wiltshire and confirmed by Ordnance Survey results that agreed with the RHB guys results (within a few centimetres).
confirmed by Ordnance Survey results that agreed with the RHB guys
results (within a few centimetres).
Well they weren’t really independent measurements. The RHB guys have bought their own GPS survey equipment, but this will be reliant on the correction data supplied by the fixed reference stations operated by the Ordnance Survey. Also the conversion from GPS ellipsoid height to Ordnance Datum Newlyn (doubtless hidden in the GPS unit’s software) is dependent on the Ordnance Survey Geoid model (OSGM02). So in effect both measurements were dependent on the Ordnance Survey, even if one set of measurements was taken by independent operators.
To get a truly independent determination of the height difference between the two hills they would have had to have used traditional levelling methods.
I’m not questioning the result, but we shouldn’t be surprised that the two sets of measurements agreed.
In reply to M1MAJ:
Fair point Martyn. Nevertheless it was a very interesting exercise and it was particularly nice to see it televised to a wider audience.
…as to Gerald’s point (that seems to have gone now) about a potential new Marilyn, he might have to get to work with his JCB at the key col for Milk Hill. It has prominence of 148m right now so it needs to find another couple of meters somewhere
In reply to MM0FMF:
Hi Andy.I was always led to believe that errors were purposly put in to GPS by the Americans so that other countries could not use them to steer guided missiles against them.What ever you measure be it a car engine cylinder bore or the distance to the moon. Everything you measure you need a tolerance band .Those 2 hills in Wiltshire were only the dfference of a sod apart so for my money they are near enough the same.Atb Geoff G6MZX
I was always led to believe that errors were purposly put in
to GPS by the Americans so that other countries could not use them to
steer guided missiles against them.
“Selective Availability” (deliberate errors in the C/A code) was switched off nearly a decade ago, but in any case survey grade GPS uses a differential technique. A network of about 100 fixed receivers at known locations monitor the signals from the satellites, and these can be correlated with data at an unknown point. This enables vastly greater accuracy than can be obtained by measuring at a single point. It could cancel out SA when it was present, and can cancel out some of the random errors that still occur (e.g. variable propagation delays).
There is still a military code which is encrypted which enables greater precision from a single point measurement.
…as to Gerald’s point (that seems to have gone now) about a
potential new Marilyn, he might have to get to work with his JCB at
the key col for Milk Hill. It has prominence of 148m right now so it
needs to find another couple of meters somewhere
Sorry Marc - a serious discussion was developing and my previous post seemed out of place and too flippant, so I deleted it.
It would be good to have more P150’s in the south, but I doubt even the RHB boys are willing to undertake the necessary topographical revisions, be it by JCB or explosives. Mind you an excavation of just 2m is very tempting. I’ll remember that when I am next looking down into a gaping hole on site which is about to be filled with mass concrete. Many excavations far exceed the 2m depth!
