While investigating the recent explosions in Tianjin, China, we noticed that the street map of China is out of alignment in both Google Earth and Google Maps. It is tempting to think that the explosions were so large that they shook the street map out of place (the largest, according to Wikipedia, has been estimated as equivalent to 21 tonnes of TNT). However, after a bit of research we have discovered that the street map offset is not new and is actually a result of old Chinese regulations from the cold war era.
It turns out that all maps that are legally created in China must use the GCJ-02 coordinate system, which according to Wikipedia, uses an encryption algorithm that offsets the map by different amounts for different locations. Google has followed the regulations and partnered with Chinese map provider AutoNavi to obtain the data and always shows the map data using the required GCJ-02 datum.
As we have mentioned in previous posts on censorship, countries can control and censor mapping information such as aerial imagery, 3D imagery and street maps that is gathered within their borders, but have little control over satellite imagery unless the company that supplies it operates from within their borders. As a result, both Google Earth and Google Maps do not show the satellite data using the offset GCJ-02 datum but stick with the standard WGS-84 datum used for the rest of the world. This results in the discrepancy we see between the street map and the satellite imagery. However, the Chinese version of Google Maps intended to be viewed from within China does comply with Chinese laws and uses the GCJ-02 datum for the satellite imagery as well. As a result, the street maps and satellite imagery line up nicely, but GPS coordinates will be offset. This is dealt with by Chinese navigation systems, which must convert between the datums to give the correct location on the map.
The China / Hong Kong border in Google Earth. The streets are out of alignment on the Chinese side, but correct on the Hong Kong side. The satellite imagery matches the latitude and longitude as produced by a standard GPS.
In the Chinese version of Google Maps, the situation is reversed. The streets are aligned with satellite imagery in China, but misaligned in Hong Kong. Both satellite imagery and street maps on the Chinese side do not match latitude and longitude as produced by a standard GPS.
Apparently Bing Maps and Apple Maps also follow Chinese regulations and use the GCJ-02 datum, whereas Open Street Map does not (and is thus illegal in China).
We also mentioned China’s strict mapping regulations back in 2006.