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Another big thing with china is that they consider all maps and satellite pics to be state secrets that require strict approval before being published. If you e

by marme 11y ago

Another big thing with china is that they consider all maps and satellite pics to be state secrets that require strict approval before being published. If you ever look on google maps, the satellite pics dont match up up the maps pics. Most are off by at least 100 ft and that is because all the map data china releases has the coordinates randomly changed by a random amount in a random direction and it is different for every city. Baidu maps does not have this problem because they actual had people sit down and manually correct all the maps to match up with the satellite pics but the gps coordinates are still wrong so when you look on your phone you are rarely ever in the correct position on the map

philwelch 11y ago
This is one of the big "wtf" moments that makes me think maybe China won't win the 21st century.
MichaelGG 11y ago
Aha! Thanks for explaining at least a little bit why sometimes I see the map road not really on the actual road. But Google Maps has people too. And you'd think they'd take GPS coords to help match up? Though fwiw I was checking out some rather rural areas.
jpatokal 11y ago
The offsets are not random, but algorithmic, using the GCJ-02 datum. Of course, legal access to use the GCJ-02 algorithms requires that you kowtow to the Chinese government: https://en.wikipedia.org/wiki/Restrictions_on_geographic_data_in_China https://en.wikipedia.org/wiki/Restrictions_on_geographic_dat...
- csours 11y ago
  > It uses an encryption algorithm[13] which adds apparently random offsets to both the latitude and longitude, with the alleged goal of improving national security. You're both right! It uses reversible randomness apparently. Wonder how hard the algorithm and seed(s) are to figure out
  - poizan42 11y ago
    > reversible randomness Yeah, we have a word for that: encryption
  - kristofferR 11y ago
    https://wuyongzheng.wordpress.com/2010/01/22/china-map-deviation-as-a-regression-problem/ https://wuyongzheng.wordpress.com/2010/01/22/china-map-devia... There are now many OSS-tools that convert between the global coordinate system standard (WGS-84) and the Chinese deliberately incompatible coordinate systems (GCJ-02 and BD-09).
sksksk 11y ago
You can see the effect on this heatmap: http://labs.strava.com/heatmap/#15/116.40337/39.96947/gray/both http://labs.strava.com/heatmap/#15/116.40337/39.96947/gray/b... There is a "ghost" trail a few meters to the east of the real trail.
janekm 11y ago
It's worth mentioning here that maps inside China don't have this problem, Baidu Maps and Apple Maps (which uses Chinese map data when used in China) for example show accurate GPS position and correlation with satellite images. It's presumably just typical government inertia at this point... Everyone using maps within China gets accurate maps so not worth the trouble of trying to change the policy.

by marme 11y ago

philwelch 11y ago

This is one of the big "wtf" moments that makes me think maybe China won't win the 21st century.

MichaelGG 11y ago

Aha! Thanks for explaining at least a little bit why sometimes I see the map road not really on the actual road. But Google Maps has people too. And you'd think they'd take GPS coords to help match up? Though fwiw I was checking out some rather rural areas.

jpatokal 11y ago

The offsets are not random, but algorithmic, using the GCJ-02 datum. Of course, legal access to use the GCJ-02 algorithms requires that you kowtow to the Chinese government: https://en.wikipedia.org/wiki/Restrictions_on_geographic_data_in_China https://en.wikipedia.org/wiki/Restrictions_on_geographic_dat...

csours 11y ago
> It uses an encryption algorithm[13] which adds apparently random offsets to both the latitude and longitude, with the alleged goal of improving national security. You're both right! It uses reversible randomness apparently. Wonder how hard the algorithm and seed(s) are to figure out
- poizan42 11y ago
  > reversible randomness Yeah, we have a word for that: encryption
- kristofferR 11y ago
  https://wuyongzheng.wordpress.com/2010/01/22/china-map-deviation-as-a-regression-problem/ https://wuyongzheng.wordpress.com/2010/01/22/china-map-devia... There are now many OSS-tools that convert between the global coordinate system standard (WGS-84) and the Chinese deliberately incompatible coordinate systems (GCJ-02 and BD-09).

sksksk 11y ago

You can see the effect on this heatmap: http://labs.strava.com/heatmap/#15/116.40337/39.96947/gray/both http://labs.strava.com/heatmap/#15/116.40337/39.96947/gray/b... There is a "ghost" trail a few meters to the east of the real trail.

janekm 11y ago

It's worth mentioning here that maps inside China don't have this problem, Baidu Maps and Apple Maps (which uses Chinese map data when used in China) for example show accurate GPS position and correlation with satellite images. It's presumably just typical government inertia at this point... Everyone using maps within China gets accurate maps so not worth the trouble of trying to change the policy.