Question 1

Why does the same place have different coordinates on different maps?

Accepted Answer

Because public maps in mainland China must use the GCJ-02 encrypted datum, and Baidu applies a further offset on top of it to produce BD09. The same building is WGS84 on GPS, usually GCJ-02 in Amap or Tencent Maps, and BD09 in Baidu Maps — typically a few hundred metres apart.

Question 2

How accurate is the conversion?

Accepted Answer

The forward offset uses the published GCJ-02 algorithm. There is no analytic inverse, so the reverse direction uses iterative refinement that converges to 1e-9 degrees (about 0.1 mm), far below real map accuracy. Note that individual providers may still add their own deviations.