WD sign

[TooTallMike]

Come on then chaps, what's this sign mean? Seen outside Trinity House in London.

 

- Mike

 

[Marmite!!]

It's an Ordnance Survey Mark :coffee:

 

Ordnance Survey (OS) is an executive agency of the United Kingdom government. It is the national mapping agency for Great Britain, and one of the world's largest producers of maps. The name reflects the original military purpose of the organisation (see ordnance and surveying) in mapping Britain during the Napoleonic Wars when there was a threat of invasion from France, and its logo includes the War Department's broad arrow heraldic mark. OS is widely regarded[citation needed] as the most systematic and thorough mapping institution in the world, detailing every corner of Britain long before satellite technology made quality maps of the same standard available elsewhere in the world.

 

Edited September 17, 2008 by Marmite!!

[TooTallMike]

Thanks Lee,

 

What's the significance of the 'No.11' ?

 

- Mike

[Marmite!!]

Thanks Lee,

 

What's the significance of the 'No.11' ?

 

- Mike

 

I would of thought that was the 11th Mark put in place & relates to the reference on their maps... You see a lot of these WD marks chiseled in the stonework of buildings around the country

[Tony B]

The key marker is at the corner of the of the Taxi park just inside the entrance of from the A4 at heathrow airport, marked by a cannon sunken in. I used to work at a place in the Cwmdu in the Black Mountains, there was an Ordnace mark on the wall of one of the buildings, that also had the elevation marked on it.

[Stormin]

This one seems to be missing the horizontal groove above the arrow. This is the main part of and ordnance survey bench mark as it is were the plate sits in to rest a level staff or prism pole on.

[john fox]

as Stormin says these are called Benchmarks and are used to identify points of known elevation above sea level

the horizontal mark ("benchmark") above the arrow indicating the exact point from which a reading can be taken

 

as benchmarks are often inscribed into buildings or things which can move over time (walls can bow outwards as well as sink:shake:) they are regarded as relatively accurate but are not exact.

 

In the days before GPS and satellites they were good enough for relative surveys but they should not be confused with Trig points (the brass inserts set into concrete posts/plints) you see on mountain tops etc which are the accurate positions from which the OS surveyors used to place in the intervening grid of benchmarks

[Tony B]

Ah that explains the 1500 on the one at Cwmdu! It's high up in the black mountains near Penyfan. the old joke was who spends the most time on Penyfan in winter? the SAS or the mountain rescue pulling them off.

[fv1609]

Didn't know I was being snapped! Caught marking my territory on the trig point at the top of Mynydd Maen in 1972.

Managed to drive the Minivan up there, you can see one of the guys for the mast with two X-Band dishes using WBFM on some klystrons modified to work in the 10Gc/s band. A very good spot for that sort of thing.

 

In those days the public display of dishes was rare & considered scary - beaming microwaves - death rays, communist, IRA (1972 was when violence really escalated) etc & I had long hair! So tried to transmit from not just high places but remote areas to minimise eagle eyed public calling the police out.