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The article implies that the source of trouble is the GCJ-02 datum. Is this really such a major problem though? In North America, both the NAD27 and NAD83 datu

by ovis 11y ago

The article implies that the source of trouble is the GCJ-02 datum. Is this really such a major problem though? In North America, both the NAD27 and NAD83 datums are still common. NAD83 nearly matches WGS84, but NAD27 can be quite off. Datum transformation is a well understood topic, so what's so special about China?

HillRat 11y ago
Yes, this. I've had to deal with inter-datum conversions in the past, and as far as I can tell, though after a very brief review, GCJ-02 is just another datum, albeit one not well documented. The "random offset" described in various documents may be easily explained by the datum being optimized for accuracy in a relatively small area, and thus errors increase as you expand your mapped area. (For those who don't speak GIS, datums are essentially a way of mapping an orange peel to a table; because any attempt to dedimensionalize points on a sphere must by necessity introduce errors, datums are designed to either minimize that error in all locations -- which is what WGS84 does -- or to gain greater accuracy by focusing on a smaller part of the globe, such as the "state plane" datums. When you see misaligned points on maps that derive data from various sources, it's usually a good idea to make sure that multiple unconverted datums aren't the source of the problem.)
- maxerickson 11y ago
  The state plane coordinate systems use NAD27 or NAD83 (plus improvements). https://en.wikipedia.org/wiki/State_Plane_Coordinate_System https://en.wikipedia.org/wiki/State_Plane_Coordinate_System The flattening of an orange peel you mention is more about projection, a datum is a mathematical description of the earth that maps out how geographic coordinates will vary. Given geographic coordinates, a projection system is needed to display them on a flat surface (or really any surface that doesn't match the shape of the geographic coordinate system). Differences between datums do arise from different modeling choices and should be accounted for when translating coordinates, but as the fine article discusses, the offsets in GCJ-02 are algorithmic and are not a refinement of WGS84, they obscure true location. https://en.wikipedia.org/wiki/Restrictions_on_geographic_data_in_China#GCJ-02 https://en.wikipedia.org/wiki/Restrictions_on_geographic_dat...
willyt 11y ago
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...

by ovis 11y ago

HillRat 11y ago

Yes, this. I've had to deal with inter-datum conversions in the past, and as far as I can tell, though after a very brief review, GCJ-02 is just another datum, albeit one not well documented. The "random offset" described in various documents may be easily explained by the datum being optimized for accuracy in a relatively small area, and thus errors increase as you expand your mapped area. (For those who don't speak GIS, datums are essentially a way of mapping an orange peel to a table; because any attempt to dedimensionalize points on a sphere must by necessity introduce errors, datums are designed to either minimize that error in all locations -- which is what WGS84 does -- or to gain greater accuracy by focusing on a smaller part of the globe, such as the "state plane" datums. When you see misaligned points on maps that derive data from various sources, it's usually a good idea to make sure that multiple unconverted datums aren't the source of the problem.)

maxerickson 11y ago
The state plane coordinate systems use NAD27 or NAD83 (plus improvements). https://en.wikipedia.org/wiki/State_Plane_Coordinate_System https://en.wikipedia.org/wiki/State_Plane_Coordinate_System The flattening of an orange peel you mention is more about projection, a datum is a mathematical description of the earth that maps out how geographic coordinates will vary. Given geographic coordinates, a projection system is needed to display them on a flat surface (or really any surface that doesn't match the shape of the geographic coordinate system). Differences between datums do arise from different modeling choices and should be accounted for when translating coordinates, but as the fine article discusses, the offsets in GCJ-02 are algorithmic and are not a refinement of WGS84, they obscure true location. https://en.wikipedia.org/wiki/Restrictions_on_geographic_data_in_China#GCJ-02 https://en.wikipedia.org/wiki/Restrictions_on_geographic_dat...

willyt 11y ago

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...