[PROJ] Seeking clarification on PROJ support for temporal transformations

[Kristian Evers]

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.

/Kristian

-----Original Message-----
From: PROJ <proj-bounces at lists.osgeo.org> On Behalf Of Nyall Dawson
Sent: 27. august 2019 22:49
To: Even Rouault <even.rouault at spatialys.com>
Cc: PROJ <proj at lists.osgeo.org>
Subject: Re: [PROJ] Seeking clarification on PROJ support for temporal transformations

On Wed, 28 Aug 2019 at 01:26, Even Rouault <even.rouault at spatialys.com> wrote:
>
> > But, if I understand correctly, WKT2:2018 would be the ultimate answer
> > to this, in that it would allow us to specify the epoch of a dataset
> > as an integral part of the data's CRS definition (which it is).
>
> Not sure to understand your "what it is", but pedantically, the epoch of the
> dataset is not part of the CRS definition. Anyway...

Well -- I think it IS a fundamental part of the definition, of equal
importance to other parameters such as false easting/northing. Without
the epoch information the coordinate data becomes meaningless, and
it's required in order to accurately position them. While it would
theoretically be possible to store this information somewhere else in
a dataset's metadata, it's such an integral part of the CRS that
shoving it right into the CRS definition is the right way forward...

Nyall


>
> --
> Spatialys - Geospatial professional services
> http://www.spatialys.com
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