All similar apps out there that are geared towards transforming WGS84 coordinates into British National Grid Reference or OSGB36 coordinates do not perform accurate conversions. Extensive testing has shown us that most of them use a Helmert 7 parameter datum transformation which, whilst good for general location purposes, the results in these apps do not offer high enough accuracy.

According to Ordnance Survey, for the transformation from ETRS89 (by all intents and purposes it is the same as WGS84 within the UK) to OSGB36 in Britain, using a single Helmert transformation will give errors of up to 3m (95%) in plan and 3.5m (95%) plan and height, depending where in the country the points of interest are. In our tests we have seen errors as high as 10m in other apps.

This has led us to develop our own solution which provides accurate transformations between WGS84 (ETRS89) and OSGB36. This allows us to produce British Grid Reference values to the full 10 figures. Our results match all the test coordinates offered by Ordnance Survey.

In order to achieve the most accurate results, we use the OS provided OSTN15 transformation model applied to approximate coordinates. More details on the algorithm and accuracy can be found in the official document from Ordnance Survey titled A Guide to Coordinate Systems in Great Britain.