- Chinese coordinates -

The use of geographic data in China is restricted, and must follow guidlines formulated by the Chinese State Bureau of Surveying and Mapping

China uses a different coordinate system from that used in the West. It is known as GCJ-02 and colloquially referred to as Mars Coordinates.

The system used in the West is WGS-84 coordinates - the ordinary latitude and longitude that we are used to.

The result is a mis-match of 100 to 200 meters in most parts of China. This can show up as giving an erroneous location when plotting a point using satellite (GPS) coordinates) on a Chinese map, or when plotting a point using coordinates from a cell phone location device on a Western app (such as Google).

Maps in China or maps using data supplied by Chinese sources are REQUIRED to use the GCJ-02 system, which results in a mismatch, unless data is properly converted.

This is especially obvious at borders, especially between Hong Kong and China, and Macau and China. Both Hong Kong and Macau use the Western WGS-84 system.

There are roads that don't connect on the map, and some that are even shown in bodies of water. Where satellite imagery is shown, it often does not overlay the map data correctly.

image from The Bizarre Relationship Between China and Google Maps

Chinese maps, however, will convert ALL coordinates to the Chinese system, and display correctly, although they may still mis-plot your satellite GPS position. This is the SAME Shenzhen/Hong Kong border area as above from Amap (or Autonavi) at http://ditu.amap.com/.

For other examples, see The Bizarre Relationship Between China and Google Maps

This map shows the relative offsets of the Chinese coordinate system, although the arrows are not drawn to scale.

image from http://www.gearthblog.com

C-code is available at https://github.com/googollee/eviltransform/blob/master/go/transform.go to convert between the two coordinate systems (WGS-84 and GCJ-02). It's NOT a straight offset, but a much more complex function, with an offset that varies depending on where you are.

Amap can be downloaded for Android or iOS.
Google play - https://play.google.com/store/apps/details?id=com.autonavi.minimap&hl=en
iOS - http://download.cnet.com/Autonavi-Has-been-updated-to-Amap/3000-12940_4-75048652.html

Google has two different maps - one using the standard WGS-84 coordinate system worldwide at Google Maps - China using Chinese data plotted without correction, and another at Google CN Maps, using the Chinese coordinate system, but with Western data overlayed ouitside of China's borders. No effort is made to correct for either system, resulting in the mismatched data at the borders.

#randyw @shadowbot