Good GPS receivers allowed changes to datums. Furthermore, there is a transformation possible between ellipsoids if you have their exact parameters.
Source: I worked at Trimble.
For a great example of this look at the border between Hong Kong and Shenzhen on plain google maps ... turn on the street map layer, then the satellite layer - they match on the HKG side of the border, but not to the north in China
This was an issue in earlier versions of Galileo Offline Maps.
It also links to https://en.wikipedia.org/wiki/Restrictions_on_geographic_dat... which says "private surveying and mapping activities have been illegal in mainland China since 2002."
So gathering data for crowdsourced maps like OpenStreetMap is illegal unless you get the Chinese government's permission.
I wonder why? And also, how they could possibly enforce it -- if you drive around with GPS logging turned on and don't tell anybody, how do they find out?
EDIT: According to https://en.wikipedia.org/wiki/Restrictions_on_geographic_dat... it "uses an obfuscation algorithm which adds apparently random offsets to both the latitude and longitude, with the alleged goal of improving national security."
True, there are probably legitimate military reasons to keep accurate maps of your territory out of the hands of potential enemies. But surely any country thinking of attacking China with, say, GPS guided missiles / bombs would also be quite capable of getting an accurate map with satellite imagery, or for that matter, a few agents carrying ordinary smartphones with GPS logging turned on...