Sea level both a simple and com­plex con­cept

Wicklow People (West Edition) - - LIFESTYLE - JIM HUR­LEY’S

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.

The 20m-high Pool­beg light­house was re­built in 1820 to re­place an orig­i­nal struc­ture erected in 1768.

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