| 6 | 6 | In a nutshell, BNG projection used to be defined against the OSGB36 datum, generated by traditional surveying methods. The UK Ordnance Survey (OS) wanted to move their mapping datum to the satellite-based datum ETRS89 (this is mostly the same as WGS84, it's just moving eastwards by 2.5cm per year or so relative to WGS84 to account for continental drift), but they wanted all their old maps (using the OSGB36 datum) to remain correct for most of their users. The solution was something called the OSTN02 transformation, which provides a non-linear transformation from eastings-northings points defined in ETRS89 to eastings-northings points defined in BNG against the OSGB36 datum. All this is is a grid of points every 1km throughout the UK at which the eastings-northings co-ordinates are defined for British National Grid projection both against the ETRS89 datum and on the old maps (essentially against the OSGB36 datum). To obtain the coordinates of a point between these postings you have to interpolate. |