In the UK we have the OSGB36 datum which OS maps and the national grid are projected from. OS maps are based on data surveyed by early Victorians with measuring

In the UK we have the OSGB36 datum which OS maps and the national grid are projected from. OS maps are based on data surveyed by early Victorians with measuring chains climbing hills and triangulating with theodolites and therefore has some errors which vary randomly across the UK. Although, amazingly, the position of Lands End relative to John O'Groats was only ~20m out it is quite complicated to convert between national grid coordinates and a WGS84 with sub meter accuracy. There is an algorithm OSTN02[0] which involves looking up a correction factor in a data set which I seem to remember is ~100 MB uncompressed. Maybe China's maps were surveyed similarly, perhaps less accurately in such a massive country, and they just don't bother to publish the correction factors? [0]https://www.ordnancesurvey.co.uk/business-and-government/help-and-support/navigation-technology/os-net/formats-for-developers.html https://www.ordnancesurvey.co.uk/business-and-government/hel...