Is the National Grid Transformation OSTN02 really as accurate as using OSGB36 control stations?
A. Let’s take the National Grid Transformation OSTN02 first. This converts ETRS89 GPS coordinates to British National Grid coordinates, and vice versa. It is a complex transformation in that the parameters it applies vary in a complex way depending on your location in Great Britain. OSTN02 now defines the National Grid such that: ETRS89 (from National GPS Network) + OSTN02 = true National Grid. In discussing the accuracy of OSTN02, it’s very important to understand exactly what we mean. The reason the OSTN02 transformation is needed is because the archived triangulation OSGB36, on which the National Grid is based, contains a complex pattern of distortions. While it is true that two adjacent OSGB36 triangulation stations are usually in agreement with each other to within a few centimetres, if we compare two triangulation stations 10 km apart by measuring a precise GPS vector, we will typically find an error of several decimetres in their relative archive coordinates. If the two stations
