Transformations and OSGM02 user guide - Leica Geosystems

The Ordnance Survey Geoid model: OSGM02Orthometric height (h) in the UK and Ireland can be found by the formula:h = H - NWhere H is the GRS80 ellipsoidal height, and N is the geoid undulation (geoid-ellipsoid separation).Please note: some publications use the notations of h and H the other way round.OSGM02 uses a grid look-up method, with geoid undulation interpolated from a 1 km grid covering the UK and theRepublic of Ireland.Similar to the Ordnance Survey (Great Britain) OSTN02 transformation, the first stage in calculating the geoid undulationis to identify in which grid cell the ETRS89 point lies. To identify the appropriate grid cell and record numbers in theGB data set follow the procedure given earlier in 'Transforming ETRS89 coordinates to OSGB36 National Grid'.To identify the appropriate grid cell and record numbers in the Northern Ireland dataset, use the following formulae:east_index = (integer_part_of (x/1000)) -550north_index = (integer_part_of (y/1000)) -800record_number = east_index + (north_index x 251) +1To identify the appropriate grid cell and record numbers in the Republic of Ireland dataset, use:east_index = (integer_part_of (x/1000)) -400north_index = (integer_part_of (y/1000)) -500record_number = east_index + (north_index x 351) +1For both Irish datasets the ETRS89 eastings (x) and ETRS89 northings (y) must be computed using the GRS80ellipsoid and ITM projection (see appendix B).Having located the correct cell, find the values of the geoid undulations at the four corners of the cell (sg 0, sg 1, sg 2, sg 3)and the offsets of the point (dx, dy) from the bottom left corner of the cell (x 0, y 0) – as shown in figure 1 above (the seand sn values are used to find the OSTN02 eastings and northings shifts).Transformations and OSGM02 user guide chapter 3v2.1 9/2002 © crown copyrightPage 25