Some countries have an interpolation grid to accurately perform the conversion between EPSG:4326 and their traditional national coordinate system. gvSIG Mobile does not use so far any of these grids. Instead, it simply uses 7-parameter transformations and the standard formulae for projections, which causes a small error when the user watches the GPS arrow on his vector files or remote raster layers (when using a datum other than WGS84).
In the case of the United Kingdom and their national grid (EPSG:27700), an online free conversion service is available via the Ordnance Survey website:
This conversion claims to have an error of only a few cm, so we can accept it as the ‘official truth’ and compare our results with theirs. With a simple loop one can create a heatmap of Britain showing the divergence between the result given by gvSIG Mobile and the one obtained from the OS service, and here is the resulting heatmap in polar coordinates:
The vector represented is [gvSIG Mobile value] minus [OS value].
For example, the argument color for London is something between blue and cyan. Since blue = 0º and green = 120º, then cyan = 60º and the argument for London is about 45º. In the modulus heatmap, London is in a middle gray, so m = 5.05 / 2 ~= 2.5 m. All this means that, for London, the X (easting) in EPSG:27700 estimated by gvSIG Mobile is about 1.5 meters bigger than the X provided by the OS service and the Y (northing) in EPSG:27700 estimated by gvSIG Mobile is also about 1.5 meters bigger than the Y provided by the OS service.
Areas in black (Dorset & Ross-shire) are the lucky counties: gvSIG Mobile’s result is very accurate. Unlucky (white) counties include Norfolk & Lincolnshire.