If the road grid in online maps of China doesn’t line up with the aerial/satellite imagery layer, Anastasia Bizyayeva explains in a Medium post earlier this year, it’s because China’s map data uses a different geodetic datum, GCJ-02, rather than WGS-84. “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.” Chinese map companies are obliged to use GCJ-02 so their maps and imagery line up; outside China, companies can choose to use Chinese data and imagery and have alignment artifacts at the Chinese border, or use Chinese data with images aligned with WGS-84 and have the roads appear offset from the imagery. [Kottke]