Mystery signs explained

Have you ever noticed a strange sign carved into the brickwork of a building or wall, often a foot or two above the ground?

These signs can be found all around the town - on public buildings, private homes, bridges and walls. They show a broad horizontol line with what appears to be an arrow pointing up to it.

In fact you will find these signs up and down the country. There are estimated to be anywhere between at least 100,000 and even up to ½m of them.

So what are they?

These signs are Ordnance Survey Benchmarks and used by the OS surveyors to confirm the height of a feature in relation to the Mean Sea-Level.

They first appeared in 1831 and continued to to be carved in to brickwork until 1993.

The Ordnance Survey 1:500 map series of 1879-80 shows the location of all benchmarks existing at that time, together with the height above sea level.

Benchmarks can be found across the town, although many were lost in the Great Demolition of the sixties, when numerous buildings were destroyed with the construction of the Relief Road. Others have been lost as buildings are replaced with newer structures.

However, plenty remain - on Shire Hall, the Port Hill railway bridge, at Castle Bridges, Cowbridge, Bengeo Street, St.Andrew Street, All Saints Church and Mill Bridge.

Ordnance Survey Bench marks (BMs) are survey marks made by Ordnance Survey to record height above Ordnance Datum. If the exact height of one BM is known, the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling.

Most commonly, the BMs are found on buildings or other semi-permanent features. Although the main network is no longer being updated, the record is still in existence and the markers will remain until they are eventually destroyed by redevelopment or erosion.

:: https://www.ordnancesurvey.co.uk/benchmarks/

The benchmark network is not longer maintained, with the system replaced by computers and satellites

A detailed look at OS Benchmarks can be found in this blog article by Anthony Rowley.