[PROJ] [gdal-dev] Static/Dynamic datum problems
even.rouault at spatialys.com
Fri Jun 21 02:26:26 PDT 2019
> I believe that the problem is that WGS84 is being used in webmapping
> situations as if it were a static datum, locked to a fixed epoch, and in
> other cases it is treated as if it were a dynamic datum.
> This has led to the situation we have in Australia where:
> WGS94 -transform-> WGS2020 (leads to a coordinate shift of ~ 1.8 metres)
> WGS94 -null transform-> WGS84 (which was correct in 1994)
> WGS2020 -null transform-> WGS84 (which will be correct in 2020)
You likely meant GDA94 (instead of WGS94) and GDA2020 (instead of WGS2020).
Yes, indeed if you look in the EPSG dataset, and transform between GDA94 and
EPSG:4326 (the "average" WGS84 CRS), you'll get a null transform, but
advertized with a 3m accuracy only, whereas direct transformation between
GDA94 and GDA2020 has 1 or 5cm accuracy depending on which method is taken.
And same for GDA2020. So there is no transitivity in chaining transformations.
The thing is that EPSG:4326 is really a not super accurate CRS, given that its
underlying datum is actually a "datum ensemble" in the modern geodetic
If you look at the candidate for the new CRS WKT revised standard
you' ll see this
ENSEMBLE["WGS 84 ensemble",
MEMBER["WGS 84 (TRANSIT)",ID["EPSG",1166]],
MEMBER["WGS 84 (G730)",ID["EPSG",1152]],
MEMBER["WGS 84 (G834)",ID["EPSG",1153]],
MEMBER["WGS 84 (G1150)",ID["EPSG",1154]],
MEMBER["WGS 84 (G1674)",ID["EPSG",1155]],
MEMBER["WGS 84 (G1762)",ID["EPSG",1156]],
And I've heard that EPSG might consider to revise their future dataset to use
that datum ensemble as the base for CRS EPSG:4326 to better reflect this
reality, instead of basing it on the datum EPSG:6326, which is not really well
So mapping to EPSG:4326 makes only sense if you don't need accuracy lower than
You might pick up one of the WGS 84 realization, and use the time-dependent
transformations I mentionned in my previous email to go from GDA94 or GDA2020
to that realization.
Spatialys - Geospatial professional services
More information about the PROJ