MAPS ARE an essential tool for anyone interested in the great outdoors and the focus of this week’s column is on altitudes as featured on maps.
Say you measure the height of a small child as being one metre; three foot three inches in imperial measure. The height of the child is 1m above the floor. If the child stands on a chair he or she is still 1m tall. What has changed is the point from which the measurement is taken; it was originally the floor, now it is the seat of the chair.
So, the fixed point from which the measurement is taken can vary but whatever the fixed point is it is known as the datum and the fixed point from which heights on maps are measured is the ordnance datum or OD.
In Ireland two ODs are commonly used to measure heights on land: OD Poolbeg and OD Malin.
The first detailed mapping of Ireland by the Ordnance Survey started in the late 1830s. The map makers had to pick an ordnance datum or OD that would act as the benchmark above which all heights would be measured. The point they picked wasn’t a very fixed one; it was the wet mark made by the sea on the wall of the Poolbeg lighthouse at low water of the spring tide on the 8th April 1837. To preserve the mark, a line was chiselled on the wall of the lighthouse.
That mark, OD Poolbeg, was the fixed point or ordnance datum above which all heights were measured on the new maps. These heights were measured in feet to one decimal point.
The Poolbeg lighthouse is located at the end of the South Wall in Dublin Bay. First built in 1768 at the end of one of the two training wall channelling the River Liffey into the bay, the lighthouse was redesigned and rebuilt in 1820.
OD Poolbeg was used on Irish maps for 120 years until it was abandoned in 1958 in favour of a new ordnance datum: OD Malin. The new ordnance datum was mean sea level measured at Portmoor Pier, Malin Head, County Donegal, and is measured in metres to two decimal points.
ODP is approximately 2.7m lower than ODM. So, to convert old ODP heights measured in feet to ODM measured in metres simply subtract 8.9 feet, that is, 2.7m, from the ODP value and multiply by 0.3048 to convert feet to metres.
Poolbeg lighthouse at the end of the South Wall in Dublin Bay