I've spent close to two full days trying to understand the nature of the infamous GPS shift in China. After sifting through the noise, two recurring symptoms are that:
- GPS coordinates (WGS-84) plot poorly on Chinese maps, with an offset ranging from 50 to 1000 meters.
- Google and Apple Maps in China have severe misalignment between their street maps and satellite imagery (live example)
Various sources make contradictory claims:
- Cold War-era regulations mandated introducing random offsets into GPS chips due to security concerns
- protectionist measures aimed to favor GPS devices manufactured in China
- China simply uses a different projection system, and the offsets are not random
- GPS chips manufactured in China offset the coordinates they return, in order to match approved Chinese maps, which must follow the GCJ-02 datum
What is the actual story? This looks like the type of pervasive problem that merits a Wikipedia page, but I haven't been able to find anything authoritative yet.
The two questions suggested as possible duplicates don't answer exactly what the cause is; rather, they describe symptoms.
Best Answer
The problem is poorly documented by authoritative sources for English speakers, despite affecting millions of people on an everyday basis. I've spend the past two days trying to understand the situation and I've created a Wikipedia article about the restrictions on mapping in China and about the China GPS shift problem. Below is the part of my research that answers the question.
The root of the problem is the severe restrictions that the Chinese State Council places on geographic data concerning China's air, land and waters. Mapping and surveying can only be done with authorization from the State Council, and foreigners must form a joint-venture in order to be granted authorization for surveys. There have been numerous examples of fines levied against individuals and companies breaking this (cough protectionist and isolationist cough) law.
Online map providers offering street maps of China must obtain an authorization from the State Council. These maps must use the GCJ-02 datum, instead of the WGS-84 that the most of the world uses. This causes WGS-84 coordinates, such as those coming from a regular GPS chip, to be plotted incorrectly on GCJ-02 maps.
The street maps displayed by both google.com/maps and google.cn/maps use GCJ-02 coordinates. This can be proved by getting the GPS (WGS-84) coordinates of a known landmark, such as the Monument to the People's Heroes in Shanghai, which is located at 31.24427 N, 121.48695 E:
Google.com Maps satellite imagery uses the WGS-84 coordinates, which causes a pretty terrible mismatch:
I'm still unclear as to whether GPS chips manufactured in China return GCJ-02 coordinates directly, or if they return WGS-84 coordinates, which approved map software can convert to GCJ-02.