Poolbeg Datum

The first levelling of Ireland was carried out between 1839 and 1843. It was referred to a datum taken to be the level of low water of a spring tide observed at Poolbeg Lighthouse in Dublin Bay on 8th April 1837. The figures derived from this first levelling were not rigorously adjusted, and when later levelling (around 1890) incorporated marks used in the original exercise some differences in value were found. All subsequent levelling up to 1952 was brought into agreement with the previous network, which was not in itself entirely homogeneous.