There are certain mysteries in life which beguile us, but we tend not to pursue their meaning.
One of these is what is that little pocket inside a pair of jeans, above the front right hand one for? Another is the black diamond mark on those annoying metal tape measures. The tape nearly cuts your hand when you let it return on its own after trying to measure your height.
But one of the strangest mysteries of all are these obscure carved signs which appear on some of our older buildings. At first they look like the sign of the ancient Carthaginians, before the Romans finished them off. But on closer inspection, the sign looks more like a three legged stool.
These are known as ‘Benchmarks’. They are used as a way of measuring height above sea level and are placed there by the Ordnance Survey. Many of us are familiar with maps from this government agency. Some of us even understand what the symbols mean, such as a church with a spire, or a tower, or even a triangulation (trig) point. But Benchmarks are a different kettle of fish altogether.
Some of these chiselled signs, usually a line with an arrow underneath, can date back as far as the early 19th century. The Ordnance Survey started to use them during mapping and surveying projects. They were used to define a precise height above sea level. This process was carried out using spirit leveling. The horizontal marks were used to support a stable ‘bench’ for a leveling stave to rest on – hence ‘benchmarks’. This design ensured that a stave could be accurately repositioned in the future and that all marks were uniform.
Unfortunately to many of us, this process sounds a bit complicated. So it was probably a good idea leaving it to the surveyors and stone masons to carry out the work of calculating how far above sea level these buildings where. At least we were safe in the knowledge Blackburn was too high up to be deluged by the sea.
There were over 500,000 benchmarks created, but this number is diminishing as roads change and buildings are demolished. The advent of Global Positioning System (GPS) has replaced Benchmarks due to its highly accurate way of measuring and coordinating height above sea level anywhere in the world. It has become the international standard for mapping and surveying.
Benchmarks may now have been consigned to the history books and old mapping archives. As is the way of all technology, there’s always something approaching round the corner to replace the current norm. But they played a valuable part in the surveying of our country and helped the Ordnance Survey to produce much needed accurate maps.
