Restrictions on geographic data in China forces all approved Chinese map providers to run a obfuscation algorithm on the WGS-84 coordinates in all parts of Chinese maps. This method, known as GCJ-02, is generally known as a "public secret" with deobfuscation methods available. However, Wikipedians often tend to record such coordinates without prior deobfuscation, leading to the equivalent of the "China GPS shift (or offset) problem" polluting Wikidata and Wikipedia itself. (Baidu has a further twist on GCJ-02 known as BD-09. Also a public secret.)
This problem can affect Wikipedia, GeoHack, and Wikidata in various means. When importing the data Wikipedians may obtain such data from commercial maps and save without checking; when reading the data in geohack, Wikipedians will find Google and Baidu map links "wrong" due to the shift. Therefore, these things should be done for all Chinese (mainland china minus South China Sea Islands where everyone is using satellite imagery) coordinates recorded:
- Tag all existing Chinese coordinates as "check required".
- Provide a tool for reviewing tagged coordinates along with deobfuscated values (original, unGCJ, unBD) on satellite imagery and on OSM.
- Remind users to check new Chinese coordinates for deviation. Perhaps just add a "check required" tag.
- In GeoHack, provide format strings for these scrambled coordinates. Probably just {lat,lon}degdec_cn{gcj,bd}.
Thanks to @Liuxinyu970226 for bringing this up in T127950#3123960. Previous discussions: on enwp, on zhwp.
The "Chinese" part mentioned above can be done simply with Google's Geocoding API's region output. It excludes MO, HK, TW, disputed territory which Google didn't buy data from Chinese sources for, and most of South China Sea islands. For a local solution, use OSM's Chinese border for land and 12nm border (without S. China Sea) for sea.
For people looking for a way to do it manually: read https://en.wikipedia.org/wiki/User:Artoria2e5/coord-notice