▻https://medium.com/@anastasia.bizyayeva/every-map-of-china-is-wrong-bc2bce145db2
GPS uses the World Geodesic Standard 1984 (or WGS-84) as its reference standard. The National Geospatial Intelligence Agency (NGA) in the U.S. maintains WGS-84. This means that de facto, most of the world has subscribed to American mapping conventions. There are a few countries that have established their own geodesic data, including Russia and China. China’s datum is called GCJ-02, which translates to ‘Topographic map non-linear confidentiality algorithm’ (the name ‘GCJ’ comes from the Chinese ‘guó-cè-jú’). Here’s where it gets interesting — GCJ-02 is based on WGS-84, but with a deliberate obfuscation algorithm applied to it. The effect of this is that there are random offsets added to both latitude and longitude, ranging from as little as 50m to as much as 500m. Going back to our map examples, this explains why sometimes street view and satellite view look aligned, while at other times they completely diverge; why sometimes the offset looks like it’s applied in a northwestern direction, sometimes south, sometimes southeast, etc. Now we know what’s happening with our map — satellite images in China differ from the GPS coordinates that we have for locations within the country because of a random offset (random direction and random amount) applied by an obfuscation algorithm. This answers the question of ‘what’ is happening, but next on our agenda is answering ‘why’ this approach is taken by the Chinese government.