Datums in Ireland

This short note gives a brief Ireland (not Northern Ireland), there mean sea level of the tide gauge at explanation about the various tidal are actually two different Ordnance Malin Head, . It was datums in use across Ireland. Datums. adopted as the national datum in 1970 from readings taken between There are three datums in common The NOC and the UK Hydrographic 1960 and 1969 and all heights on use - Mean Sea Level, Chart Datum Office still use “Ordnance Datum national grid maps are measured (ACD) and Ordnance Datum (ODx, ”. What makes ODD unusual above this datum. where x is the initial letter of the in that it is the only ordnance datum reference location). based on the lowest level the tide OD (Malin) is actually the same reached in a given period rather than vertical level as OD (Belfast). If you have a datum value of x a mean level over a given period. metres, you first need to check Therefore for most locations it lies whether this is referring to the below chart datum (hence the difference between ACD and MSL, reason why you must be careful to or the difference between ACD and check whether the value is positive OD. You also have to be careful or negative). about the signs (+ or -) used when given this difference in level. It is ODD (Ordnance Datum Dublin) also usually safer to say a datum is x known as metres above or below another Datum, is defined as the level to datum than to just specify positive or which the tide fell on 8 April 1837 in negative numbers.. . The Ordnance Survey Bench Mark in Each country usually has its own There is another datum called Malin Belfast Ordnance Datum. In the case of Head Vertical Datum which is the

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