Mapping China - Chinese style

[Randy W]

GPS offset in data for China

I think the guy here misses the mark for a very obvious reason - Google and some others get their map data from Chinese sources with the offset already built in and doesn't bother with the correction - but there are still some interesting observations and things to be aware of.

The Bizarre Relationship Between China and Google Maps

The offset shows clearly here between Hong Kong and Shenzhen. San Sham Road in Hong Kong connects directly to XinShen Rd. in Shenzhen, but, due to the offset applied to the Chinese side, well, it's off by a couple hundred yards or so.

This same offset shows up in Google maps anywhere in China, and can show up as discrepancies between satellite data and map data.

The best maps to use are ones which use their own data, or have the correction accounted for - Autonavi or Amaps is a good source at http://ditu.amap.com/. They are the most up-to-date - I believe they are the ones who supply the map data to Google - and even have their own phone app similar to Google maps (but which will show your correct location).

The difference between Autonavi andGoogle is that Google data is older, and doesn't use the correction

Edited June 3, 2021 by Randy W (see edit history)

[Randy W]

This is apparently the C code that converts from one coordinate system to the other (WGS-84 and GCJ-02, the Chinese system). It's NOT a straight offset, but a much more complex function, with an offset that varies depending on where you are.

 

https://github.com/googollee/eviltransform/blob/master/go/transform.go

 

The Chinese maps I've looked at online seem to handle this issue seamlessly - Google and Bing do not.

Edited December 12, 2015 by Randy W (see edit history)

[Randy W]

This map shows the relative offsets due to the Chinese coordinate system

 

Edited June 3, 2021 by Randy W (see edit history)

[Randy W]

 

The Western coordinate system is called WGS-84, while the Chinese system is called GCJ-02. Google plots data and features without conversion from one system to the other. The confusion arises from plotting data from conflicting systems on the same plot. Chinese maps (Baidu and Amaps) seem to avoid this.

Restrictions on geographic data in China

GCJ-02
GCJ-02 (colloquially Mars Coordinates, officially Chinese: 地形图非线性保密处理算法; lit. 'Topographic map non-linear confidentiality algorithm') is a geodetic datum formulated by the Chinese State Bureau of Surveying and Mapping (Chinese: 国测局; pinyin: guó-cè-jú), and based on WGS-84. It uses an obfuscation algorithm which adds apparently random offsets to both the latitude and longitude, with the alleged goal of improving national security. There is a license fee associated with using this mandatory algorithm in China.[21]

A marker with GCJ-02 coordinates will be displayed at the correct location on a GCJ-02 map. However, the offsets can result in a 100–700 meter error from the actual location if a WGS-84 marker (such as a GPS location) is placed on a GCJ-02 map, or vice versa. The Google.com street map is offset by 50–500 meters from its satellite imagery,[11][25] while the Google.cn map is not. Yahoo! Maps also displays the street map without major errors when compared to the satellite imagery.[27] MapQuest overlays OpenStreetMap data perfectly as well.