China datums
Datums in China
Please see e.g., http://www.asprs.org/a/resources/grids/05-2000-china.pdf and https://www.iag-aig.org/doc/5d8dd34ba63c6.pdf. On this page we aim to just supplement the more widely known information.
If you are doing modern work, you should only see CGCS2000 (EPSG:4490), which is close to ITRS and WGS 84 for casual use. Parameters for precision transforms is not officially given, but you can try your luck with Wang et al. (2019), Table 1.
Transformation for PRC datum are only legally acquired from the local government, which should normally be your source for maps made in old datums too. For Beijing 1954 (EPSG:4214), the EPSG has a few local transforms to try. All articles dealing with Xian 1980 are heavily redacted; the screenshots in 10.32629/gmsm.v2i4.291 look promising, until you realize the Bursa-Wolf values look way off and the source data is also redacted.
- Artoria2e5: On a second look, the redacted source numbers actually seem plausible and they are done sloppily enough to maybe recover the original. Although three control points will only give a small useful area (this could also explain the nonsense 7 parameters), it's something. The PDF is archived at [1] (click "Download Zip") for anyone who wants to try. --Artoria2e5 (talk) 07:59, 17 May 2023 (PDT)
Coordinate systems used in China
Web map examples
Some common coordinate systems observed, 觀測中國電子地圖諸網站常見座標系統，
Here we give examples of how a single longitude latitude pair is located differently depending on which web mapping site one uses!
This is necessitated by the state mandated distortion... 其政府所規定之偏移…
- http://maps.google.com/maps?q=loc:40,116.5&t=k
- http://maps.google.com/maps?q=loc:40,116.5&t=m
- http://api.map.baidu.com/staticimage?markers=116.5,40&zoom=14
We observe we can obtain the exact same three points,
- http://map.sogou.com/#hb=1,1&c=12968861,4838218,15&tip=1,12968861,4838218
- http://map.sogou.com/#hb=1,1&c=12968189,4838039,15&tip=2,12968189,4838039
- http://map.sogou.com/#hb=1,1&c=12967462,4837158,15&tip=3,12967462,4837158
via 搜狗 Sogou API's type=1, 2, 3. We also notice type=1 is a plain Mercator projection, Clarke 1866 ellipsoid, a=6378206.4 rf=294.978698213898 .
$ proj +proj=merc +ellps=clrk66
(Note not all sites are actually reachable from inside China.)
See the Wikipedia page on GCJ-02 for how and why the map distortion is done. On top of messing with the lat/lon, each site uses their own variation of Mercator for tiling, which is less than convenient. cntms is a Python map tile proxy with support for some Chinese sites: you will see that Baidu also uses Clarke 1866 Mercator.