[PROJ] Seeking clarification on PROJ support for temporal transformations

[Even Rouault]

On mardi 27 août 2019 20:12:27 EEST Kristian Evers wrote:
> Nyall,
> 
> The epoch of a coordinate, e.g. the time it was measured, has nothing to do
> with the CRS definition.
 But it is important once you want to transform
> from one CRS to another or propagate the coordinate through time in the
> same CRS. Coordinates in a dataset will not necessarily have the same
> measuring time/epoch, which is why it is not a good idea to store that
> information alongside the CRS description. This is why 4D coordinates are
> necessary for dynamic CRS's. You need a XYZT geometry, as it were.
> 
> There are of course exceptions, for instance a raster image where all pixels
> are captured simultaneous by the sensor. For that it would be nice to have
> a way to register the capture time in a standardized way. I am not sure if
> this is already possible with WKT2:2018.

The COORDINATEMETADATA[] WKT2:2018 construct I showed in the first reply to 
this thread would be a way to do that. That construct is not acknowledged as a 
CRS by itself but a pair consisting of a CRS + a coordinate epoch

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