The Sappers Mark Traditional Geocache

Sappers Mark aka Crows feet.
Ireland is covered with crow’s feet. They are everywhere - indented in stone on walls, pillars, steps – in towns and villages all over the country. Once important bench-markings to facilitate drainage and engineering projects, giving a value of ; a height above sea level. With modern mapping technology they are now superseded and obsolete. At the commencement of the first large scale Ordnance Survey maps of the country a national datum for levelling was established at Poolbeg lighthouse in Dublin port. 

The original mark set at the base of the Poolbeg Lighthouse in Dublin port was set at low water ordinary spring tides on 8th April 1837 and found to be 20ft 11in below a permanent mark on the base of the lighthouse tower. From this “zero mark” level lines were run, primary, secondary and tertiary to traverse the country and establish heights. (The datum was changed to mean sea level at Malin Head, Co. Donegal in 1958.)
These marks cut by the “Sappers” or Levellers (in the early 1800s) were cut by hand on sights that were selected with a view to permanency, therefore these benchmarks were indeed written in stone. 

Cold chisels for cutting them had to be sharpened and finely tempered and although forges were petering out there were still a scattering of elderly men with furnace, hammer and anvil up to the task. 

To cut a mark a levelling bracket was placed against the stone, positioning it with the plate bubble and then after further small spirit level adjustments the three inch line was marked on the stone and then cut out. 

Quote from a Leveller on the procedure:
 “First lightly tracking the stone with a newly sharpened and tempered chisel, I then carefully deepened the crevasse, occasionally checking that the metal flange of the levelling bracket fitted snugly within the cut. Then satisfied it was level and true the ornamental legs of the distinctive ‘crows foot’ could be marked and their indenting began. I had no idea that in a short number of years the whole operation would be superseded by readings from navigational satellites in the sky.“