OS bagging

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Re: OS bagging

Postby RickH » 1 Jan 2019, 6:45pm

We went for a walk down most of the Wigan locks on the Leeds-Liverpool Canal on Boxing Day. I'd not checked for locations of benchmarks beforehand & spotted precisely zero! :? Looking at the 1894 OS 25 inch map afterwards I counted 19 marks that we walked past the location of. Some will have disappeared - bridges widened, buildings demolished, etc. - but I thought I would have found at least one.

On Friday we took the grandchildren to see the (not as famous as I think it should be :D ) Meccano Bridge at the head of Prestolee Locks on the old Manchester, Bolton & Bury Canal. Here I spotted one in the wall as we walked down next to the flight of locks. I've not found it marked on any maps I've looked at but it is in the OS database linked to earlier.
IMG_2481_e (Small).jpg
Meccano Bridge (click to enlarge)
IMG_2483_e (Small).jpg
Benchmark near locks (click to enlarge)
IMG_2485_e (Small).jpg
Benchmark is in wall to left, Bridge is in distance on right (click to enlarge)

Manchester Bolton & Bury Canal Society have a similar photo, but taken from the other side of the channel showing the original bridge
(From MB&BCS Key Facts page)

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Re: OS bagging

Postby Lance Dopestrong » 31 Mar 2019, 10:01pm

Nice work Rick. I must pull my finger out and bag a few more.
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Re: OS bagging

Postby gaz » 25 Nov 2019, 11:13am

It's a bit damp out. Remembered this thread whilst I was passing.



Re: OS bagging

Postby DNC123 » 25 Nov 2019, 11:34am

[Anorak mode on]
How many on here know that Mean Sea level changed in 1921? Prior to that date it was measured from a mark in Victoria Dock Liverpool. After 1921 the measurement was taken from Newlyn in Cornwall and sea level is apparently different down there. Maybe it's to do with the curvature of the earth!!! Whatever all heights changed when the datum point was moved.
[/Anorak mode off]

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Re: OS bagging

Postby brynpoeth » 25 Nov 2019, 12:51pm

We know that now, very good, else one should never have known of Newlyn
Sea level is expected to rise so mountains shall become lower
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Re: OS bagging

Postby arnsider » 25 Nov 2019, 6:41pm

Ordnance Survey down graded the whole Trig Network when GPS became really accurate around the early nineties.
Before then, a colossal operation was undertaken to establish the network, with first order levelling and Theodolite observations.
Actually finding old marks can be a needle in haystack scenario, especially bench marks on walls.
I would not have thought the pursuit to be compatible with cycling bearing in mind the ferreting about required off the bike!

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Re: OS bagging

Postby arnsider » 25 Nov 2019, 6:50pm

Newlyn observatory determined Mean Sea level over a long period.
Mean sea level is pretty dynamic and is subject to great variation, storm surges, tidal stream velocity etc.
OS levels were suppied predominantly for engineering purposes namely drainage and as such were adequate.
Geodetic considerations are a seperate field.

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Re: OS bagging

Postby arnsider » 25 Nov 2019, 6:57pm

The flush brackets were designed to take a metal platform that had a foot at the same level as the arrow top.
The same accuracy could be achieved sitting the staff on a two bob coin, provided you allowed a few millimeters for the coin thickness.
I well remember closing a long line of obs on a bracket on a church near Droitwich. The sums misclosed by a couple of feet and I was crestfallen at the prospect of doing it all again.
I carried on to the next OSBM and hey presto, it closed within thirty millimeters. Some mason had moved the bracket up the wall a couple of courses!!!

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Re: OS bagging

Postby Will » 28 Nov 2019, 1:11pm

arnsider wrote:Some mason had moved the bracket up the wall a couple of courses!!!

I've seen that before. Many years ago I was a Land Surveyor. One day we had to do a spot of levelling and went to the nearest BM to our site. It looked like the wall had been rebuilt. We closed on a couple of other BMs to verify. There was a new road being constructed next to our site and as we were packing up a Civil Engineer came over to warn us about the local BM. Apparently they had come off it without checking it. They had to make some 'adjustments' on the road construction in order to get it to meet in the middle :oops:.

The National Library for Scotland Maps website (https://maps.nls.uk/) has 1:1250 and 1:2500 maps that show BMs.


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Re: OS bagging

Postby arnsider » 6 Dec 2019, 12:24pm

Dare I say that the standards of Surveying and levelling practises among Civil Engineers leaves a lot to be desired.