OS Benchmarking - Distances Above Sea Level

RECENTLY I DISCOVERED Ordnance Survey Benchmarking and for some reason it has got me really interested in these historic markings that can be found all over the place.   If you live in the UK, you have passed these symbols thousands of times and never noticed them.   They are called benchmarks and nobody knows exactly how many there are.   It doesn't matter how remote you live you will have these hidden around you in plain sight.

They were installed by Ordnance Survey, Britain's national mapping agency starting in the 19th Century.   If you wanted to know the difference in elevation between your town and the next one, there were very sophisticated tools that would allow you measure it, but you would have to go out and work it out yourself because no one else had done it for you.

As we started to build things like railways, it was becoming more important to know the difference in elevation across the country.   Ordnance survey went out and spent two decades mapping out lines across Great Britain and Ireland, measuring the differences in elevation along the way.  At ever point they measured, they left a bench mark embossed into brickwork.

Once that was done, each of these benchmarks became a new reference point that could be used to fill in the rest of the country.   If you look at ordnance survey maps from the 19th you will see these bench marks everywhere.   This means that if you want to know the height of a particular point, all you had to do was measure it relative to the nearest benchmark.   This would give you its height relative to every other part of the country.

These days with GPS, there's not much use for benchmarks and they were decommissioned 30 years ago.    But there is still one small part of the network that's still in use today.    They are called "fundamental benchmarks", and there are about 200 of them.    They were built more than a century ago to serve as a solid bedrock for the rest of the network.   They will go almost two metres underground.

These days they contain passive GPS stations.   Since we know the exact position of these benchmarks, we can use them to correct and enhance the data we receive from GPS satellites.   And using this, modern surveyors can pinpoint their location anywhere in the country to an accuracy of about a centimeter.

The benchmarks live on but more of them are being lost each year due to redevelopment.

I was stopped in Reigate by a local man asking what I was looking for, so I explained it to him.   He understood exactly what I was talking about and told me there are two types, the ones with the line above the arrow which is ordnance survey and ones without the line above which is military.    Some of them are brass plates.

When I go out on my runs, I will be keeping an eye out of these, looking them up and seeing what the height is that has been recorded for them.    I know I am looking for public buildings and walls over 30 years old.

The hunt begins ....