28-07-2025
The story behind the benchmarks hiding in plain sight around Ireland
Analysis: The Ordnance Survey benchmarks chiselled into buildings, walls and bridges were originally used to measure height across Ireland
Ordnance Survey (OS) benchmarks, also known as crow's feet, have marked our cultural landscape for nearly two centuries. They're often hidden in plain sight in our towns, cities and rural parts of Ireland, but do you know their purpose and what do they mean to us today as heritage?
On April 8th 1837 during low water spring tide, the OS took a height measurement at Poolbeg Lighthouse in Dublin Bay. This fundamental reference point of 20.90 feet became known as the Irish Ordnance Datum. From here, between 1839 and 1843, the OS established the Principal Spirit Lines of Levelling, measuring height and inscribing 'bench marks' into the fabric of the island. The first of these ran from Poolbeg, through Dublin to Monasterevin along the Grand Canal, followed by an expanding network of spirit lines across the country.
How did they take measurements?
Teams comprising Royal Sappers, Miners, civil assistants and typically three labourers measured height across the country. Two labourers managed staves and lay chains, while another carried the equipment and provided shelter with an umbrella.
Following major transport routes, teams recorded approximately five spot height measurements per square kilometre in rural areas, with more frequent readings in urban environments. Stone structures along roads were favoured for bench mark placement, though in remote locations, field stones or even wooden posts were used instead. By law, surveyors had the right to enter any property to take measurements.
Surveyors used a cold chisel to carve a horizontal line typically one to two feet above ground into vertical surfaces or on horizontal faces such as quays. An angle iron was inserted to form a 'bench', upon which a levelling rod was placed and observation and height measured relative to a nearby known 'spot height'. Alternatively, a finger sized hole was cut instead and occasionally fitted with a rivet or bolt.
Branch or side levels were extended from the main levelling routes, additionally using nearby trigonometrical stations to determine height. Surveyors employed standard backsight and foresight measurements at intervals logging the height in their field notebooks before dispatching them to Dublin. To maintain accuracy and prevent collusion, each line was measured twice by separate teams moving in opposite directions.
A broad arrow, typically four to six inches across, was carved beneath each levelling line to help future surveyors relocate it. The bench mark locations later appeared on published OS maps, identified by a crow's foot symbol, their height in imperial feet, and occasionally accompanying descriptive text. This ensured open access to height data for anyone who required it.
What do they look like and where can you find them?
Benchmarks vary in type, form and quality depending on where, when and by whom they were placed. Most are 'cut' marks, known colloquially as a crow's foot or sappers' mark, and featuring a characteristic upward pointing 'broad arrow' (or inverted pheon) beneath the horizontal line cut or indentation. This was originally used by the British Government to denote ownership of property (from objects and buildings to animals and even people!) and was adopted by the OS as a recognisable and easily carved symbol.
Primarily chiselled into the vertical stonework of buildings, bridges or walls, benchmarks also appear on horizontal surfaces such as doorsteps, windowsills and quays, and sometimes taking alternative forms such as plates, brackets, rivets, bolts and spikes.
What is the heritage value of benchmarks?
Although benchmarks were cut for over 150 years, they were superseded in the 1990s by modern digital mapping techniques. Today, they serve as tangible links to the past, embodying a legacy of scientific ingenuity and progress in Ireland and beyond and over nearly two centuries they have secured a place in Ireland's fabric and folklore.
Yet their heritage is complex. Rooted in British military history, benchmarks and particularly the broad arrow, can be seen not only as tools of measurement, but also through the lens of contested heritage as exhibiting authority and ownership. Over time, many have been vandalised or removed, and examples of heritage erasure remain.
This tension is captured alongside folklore in a meeting recalled by surveyor Robin Riddihough in OSi News (1998). While working in Co Mayo in the mid 20th-century, he encountered an elderly lady who referred to them as the 'Devil's' mark '. "I remember when I was a little girl seeing figures up on the hillsides. My father told me they were sent by the Devil and wherever they went they left an arrow cut into the stone. When they were gone, we were told to smash the marks so that the Devil could not return. You'll not find those marks around these parts!"
Benchmarks are now recognised as remarkable feats of scientific engineering that helped shape our understanding of Ireland's terrain. They were instrumental to planners and engineers in construction and continue to offer crucial insight into topography across the island. In the face of a rapidly changing climate and rising sea levels, these marks provide valuable data for tracking change over time.
However, OS benchmarks in Ireland lack protected status and many have been disappeared over time due to demolition, redevelopment and environmental change. Their total number was never recorded and therefore the extent of this loss remains unknown.
They undoubtedly carry a complex legacy tied to Ireland's colonial past, but the early Ordnance Survey's pioneering work laid the foundation for modern mapping and surveying and significantly advanced understanding of Ireland's landscape. There is an urgent need for dialogue on protecting and preserving this rapidly disappearing heritage.
