Height above sea level is both a simple and a complex concept. At the simple level, the height of a sea wall can easily be measured as so many metres above the apparently flat surface of a calm sea.
However, the surface of the sea is curved rather than being flat and its level rises and falls with both flooding and ebbing tides and the monthly cycle of our solitary orbiting Moon.
In the 1830s, it was decided to fix sea level as the wet mark that the sea made on the foundation wall of the Poolbeg lighthouse at low water of the spring tide on the 8th April 1837. To preserve the mark on the iconic red building at the end of the South Wall in Dublin Bay, a line was chiselled on the rock. All measurements of height in Ireland were subsequently made in feet above the Poolbeg chisel mark. That system prevailed until it was abandoned in 1958.
In the 1950s, it was decided to fix sea level as mean sea level at Portmoor Pier, Malin Head, County Donegal, and to henceforth measure heights in metres and millimetres above the Malin reference level. As a national ballpark figure, the new Malin level was 2.7m above the old Poolbeg level. Mean sea level at Malin Head, the most northly point of the island of Ireland, was calculated from a series of measurements taken between 1960 and 1969.
The world is not round. It is flattened at the poles, bulges in the middle and has various humps and bumps so its rocky surface it is more like that of a baked apple than a billiard ball. When all the unevennesses are averaged out the shape in cross-section is described as a distorted oval rather than a circle. The overlying shape that mean sea level takes, ignoring the not insignificant influences of tides, wind and weather, is called the geoid.
Unlike the line chiselled on the wall of the Poolbeg lighthouse, the geoid is not a physical entity; it is a mathematical model in our brains and in computers. Using signals from an array of orbiting satellites, instruments measure heights above sea level to decimal points of a millimetre compensating for location, distance from Malin Head, the tidal cycle and anomalies in the strength of the pull of gravity.
Advances continue to be made: OSGM02, the former geoid model, is now being replaced by the more accurate and updated OSGM15.