Guided by crows' feet

CURIOSITIES: IF YOU ARE at all observant, then you have probably seen this "crow's foot" or benchmark carved into a wall somewhere…

CURIOSITIES:IF YOU ARE at all observant, then you have probably seen this "crow's foot" or benchmark carved into a wall somewhere. And if you are at all curious, then you probably wondered at its significance.

Hillwalkers will certainly be familiar with the sign, as it can be found on the "trig" point on many a summit, and this gives a clue to its purpose: part of the ordnance survey of Ireland, begun by the British military in the 1830s, and aimed at producing an accurate map of the country for a new rateable valuation and taxation system.

Crows' feet once covered the country, etched discreetly into walls and structures every few hundred metres, as if some weird flock of birds with chisel-sharp claws had taken to scratching at stones.

The marks were part of a low-tech yet sophisticated system that enabled the ordnance surveyors to map the terrain. For this is, literally, a benchmark: where a surveyor could sit their "bench" and determine height above sea level.


The baseline mark was made on April 8th, 1837, when a team of surveyors recorded the low water mark of the spring tide at Poolbeg Lighthouse in Dublin Bay. And for more than 100 years, all the spot heights on Irish ordnance survey maps were calculated relative to that mark, known to the cognoscenti as Ordnance Datum (or OD) Poolbeg.

But, a single record is not a very accurate measurement of sea level. So, in 1959, OD Poolbeg was replaced by a new datum, based on 10 years' measurements of average sea level at Malin Head in Co Donegal.

The difference between the two systems is 2.7m, and explains why heights on older Irish maps seem higher than today.

For a century, the elegant yet simple crow's foot benchmarks were protected structures, of such national importance that they could not be moved. Today, however, they've been superseded by modern instruments and satellite technology and, being no longer protected, are often moved when a wall is rebuilt or demolished.

But the sharp-eyed among you will still be able to spot them on old stone walls and, at low water on a spring tide, at Poolbeg lighthouse.