Na­ture Trail

Bray People - - NEWS - WITH JIM HUR­LEY

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

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