Well I have given up ,I imagined I would take two places the same height above sea level,Armagh would not be a good second choice as we are too near ,then calculate distance angles etc. that is far above my level of maths now given that it is fact that the sea levels are not the same .One interesting fact ,there is a mean sea level used for calculating heights especially for the needs of aircraft ,there is a place in the middle of the Indian Ocean that is 106 metres below the global mean . The MSL for the UK was calculated from the Victoria dock before 1921 ,so perhaps BMD by association !