Datum Transformation

A datum transformation converts coordinates from one geodetic datum to another so that positions referenced to different frames overlay correctly. A point on a local datum such as ED50 or OSGB36 plots tens to hundreds of meters from its WGS 84 position, so the shift must be modeled. Common methods are the three-parameter Molodensky shift and the seven-parameter Helmert (Bursa-Wolf) transformation, which applies three translations, three rotations, and a scale factor, plus grid-based methods such as NTv2 for higher accuracy. EPSG codes identify each transformation and its accuracy. Failing to apply the right transformation is a frequent cause of chart and survey position error.