Coord Convert

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Coordinate conversion utilities for the Chinese map coordinate systems, implementing the standard algorithms used by China's surveying authorities.

Why this exists

Chinese online maps are required by law to render coordinates with a non-linear offset applied — GCJ-02, colloquially "Mars coordinates". Raw GPS data (WGS-84) therefore does not line up with Amap, Baidu, or Tencent imagery, and mixing the two silently shifts points by several hundred metres. This toolkit makes the conversion explicit, verifiable, and reversible.

Supported systems

System	Description	Used by
WGS-84	Standard GPS coordinates	International systems, offline maps
GCJ-02	Obfuscated "Mars" coordinates	Amap, Google China, Tencent
BD-09	Baidu coordinates (GCJ-02 plus a second offset)	Baidu Maps

Tools

File	Purpose
gcj2wgs_converter.py	GCJ-02 → WGS-84, supports single points and batch conversion
amap_wgs84_loader.py	Loads Amap tiles in QGIS with real-time correction to WGS-84
test_coord_conversion.py	Algorithm verification and error measurement

Usage

# Convert GCJ-02 to WGS-84
python gcj2wgs_converter.py

# Run algorithm verification
python test_coord_conversion.py

Code example

from gcj2wgs_converter import gcj02_to_wgs84

# An Amap coordinate (GCJ-02)
gcj_lon, gcj_lat = 114.305202, 30.625309

# Convert to standard GPS coordinates (WGS-84)
wgs_lon, wgs_lat = gcj02_to_wgs84(gcj_lon, gcj_lat)
print(f"WGS-84: {wgs_lon:.6f}, {wgs_lat:.6f}")

QGIS tile correction

amap_wgs84_loader.py builds an Amap vector-tile layer inside QGIS that is reprojected from GCJ-02 to WGS-84 at load time, so the basemap can be overlaid on standard GPS data without manual offsetting. Run it from the QGIS Python console.

License

MIT © Retter Wang