Transformations and OSGM02 user guide - Leica Geosystems

The transformation is based on 183 points evenly distributed throughout Ireland and Northern Ireland. The preciseETRS89 and Irish Grid coordinates of these points are determined by GPS and terrestrial survey methods, and aone-dimensional 3 rd order polynomial individually fitted to the latitude and the longitude. The resulting polynomial allowscalculation of the coordinate differences at additional points. The polynomial transformation has an accuracy of 0.4 m(95% data).Ordnance Survey Geoid model: OSGM02To provide the third dimension of the transformation, the Ordnance Surveys have, with others, developed the Geoidmodel OSGM02. The model is derived from precise gravity surveys across UK, Ireland, and surrounding waters;additionally, the model includes data from the global geopotential model (EGM96). Alignment to each regional verticaldatum is based on precise GPS observations at Ordnance Survey levelling marks. Within Great Britain these include theOrdnance Survey fundamental bench mark network.The Geoid model consists of a 1 km grid with geoid-ellipsoid separation values covering all of Great Britain, Ireland andNorthern Ireland. This model can be used with GPS determined positions to establish height above mean sea level, asdefined by the respective vertical datums, to the accuracies shown in the table below. The Ordnance Surveysrecommend the use of the Geoid model OSGM02 and the active GPS network to produce orthometric heightcompatible with Ordnance Survey mapping. The standard error of the main datums are:OSGM02 regionStandard error (m)Great Britain 0.02Republic of Ireland 0.03Northern Ireland 0.02Orkney 0.08Shetland 0.03Outer Hebrides 0.09Isle of Man 0.03St Kilda 0.06Transformations and OSGM02 user guide chapter 1v2.1 9/2002 © crown copyrightPage 8