The return levels presented in this report are generally relative to the main tide gauge benchmark, which has a fixed height relative to Ordnance Datum Newlyn (ODN) and Ordnance Datum Belfast (in Northern Ireland). Sites relative to local datums include Stornoway, Lerwick and St Mary’s.
ODN is referred to here as the height above mean sea level at Newlyn from 1950 to 1968. Comparisons of the return levels can only be made with other data relative to the same datum (that is, ODN). Heights measured using a GPS can be converted to ODN via the spatial surface of the transformation model OSGM15, or previously OSGM02.