[gdal-dev] Motion: adopt RFC 81: support for coordinate epochs in geospatial formats

Even Rouault even.rouault at spatialys.com
Thu May 27 11:19:25 PDT 2021

Regarding KML and GeoJSON and OGC:CRS84 (or EPSG:4326 since they are the 
same thing, except axis order), that's a good and hard question. 
Actually that extends to *any* CRS built on top of them, like all the 
EPSG:32[6|7][01-60] UTM CRS, and that's probably for those later than 
things are the most problematic since there's no EPSG code for a UTM WGS 
84 (G1762) CRS. I guess people would potentially want to attach a 
coordinate epoch to them, and have a (non-encoded-in-the-format) 
convention that they actually mean WGS 84 (G1762) (or possibly an 
earlier realization. The datum ensemble reality of WGS 84 is really due 
to Transit which differs significantly from later realizations, which 
are pretty consistent between them within a few centimers) . So I 
wouldn't forbid such uses (I actually got private feedback that some 
people would even want to store coordinate epoch for CRS that aren't 
considered by EPSG as dynamic CRS, but which, for advanced uses, can 
still have things like deformation models, like NAD83(2011) that is used 
for most people as a static CRS, but is more a snapshot of a dynamic CRS 
with a conventional reference epoch of 2010.0).

That said, I'll probably drop the commit for KML as it is clearly a 
hack, but I think support for GeoJSON, which is cleaner, could still be 
useful at some point as it is widely used as an exchange format. But 
I'll defer for GeoJSON until we see if the /OGC Features/ and 
/Geometries JSON/ SWG comes with something regarding this.


Le 27/05/2021 à 19:24, Sean Gillies via gdal-dev a écrit :
> Hi all,
> I've got a suggestion about limiting the number of formats.
> GeoJSON and KML don't need support for a coordinate epoch. Both of 
> these are pretty cleared intended for low accuracy data (1-2 meters). 
> KML and GeoJSON don't support any CRS other than OGC:CRS84, which uses 
> (it has been pointed out many times) a datum ensemble that RFC 81 does 
> not intend to address. Right, Even? So let's leave these formats the 
> way they are.
> GML, GeoPackage, PostGIS, and the actively used raster formats like 
> GeoTIFF seem like the important ones to me.
> On Thu, May 27, 2021 at 7:40 AM Even Rouault 
> <even.rouault at spatialys.com <mailto:even.rouault at spatialys.com>> wrote:
>     Hi,
>     - merging the underlying API without any format support is I
>     believe of
>     little interest. So I'll wait for at least one format (likely
>     GeoPackage) to have merged in the master of their specification an
>     official way of storing the coordinate epoch. I've also prepared an
>     enhancement of the GeoTIFF spec regarding this
>     (https://github.com/opengeospatial/geotiff/pull/99
>     <https://github.com/opengeospatial/geotiff/pull/99>) that will
>     likely be
>     discussed at the next OGC GeoTIFF SWG meeting.
>     - I've just chatted with Howard and a good compromise could be
>     that for
>     formats that will have an official way of storing the coordinate
>     epoch,
>     we store it by default (when it is set), and for formats that we
>     unilaterally extend (GML, KML, GeoJSON, etc.), we require an explicit
>     GDAL_ENABLE_COORD_EPOCH_STORAGE=YES configuration option to be set
>     (default would be NO). We might revisit in the future the default
>     value.
>     - On the vector side of things, things will probably get more
>     complicated for some drivers, as it is likely that Spatialite (see
>     https://groups.google.com/g/spatialite-users/c/NtbuBpRzYBE
>     <https://groups.google.com/g/spatialite-users/c/NtbuBpRzYBE>) or
>     PostGIS
>     might have per-geometry coordinate epoch, not at the layer level.
>     But I
>     don't think that would invalidate having support for it at the layer
>     level (those database already support a per-geometry CRS, but GDAL
>     support for that is probably lacking a bit in those drivers), as a
>     OGRGeometry can potentially be associated with its own
>     OGRSpatialReference object.
>     Even
>     Le 27/05/2021 à 15:01, Howard Butler a écrit :
>     >
>     >> On May 26, 2021, at 8:33 PM, Nyall Dawson
>     <nyall.dawson at gmail.com <mailto:nyall.dawson at gmail.com>> wrote:
>     >>
>     >> Can I make the suggestion that a subset of
>     >> https://github.com/OSGeo/gdal/pull/3827
>     <https://github.com/OSGeo/gdal/pull/3827> could be created and be
>     merged
>     >> on its own? Specifically the commits which add the underlying
>     API for
>     >> GDAL to handle epochs should be controversy-free and suitable for
>     >> merge outside of the larger/trickier question of patching in
>     support
>     >> to the data formats.
>     > :thumbsup:
>     >
>     > As for the patching of data formats with GDAL
>     application-specific metadata, as I said, I don't have a better
>     option, but I'm satisfied if the process of writing epochs into
>     metadata is opt-in (some kind of global CRS option? we already
>     have magic switches like that).
>     >
>     > Howard
>     >
>     >
> -- 
> Sean Gillies
> _______________________________________________
> gdal-dev mailing list
> gdal-dev at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/gdal-dev

My software is free, but my time generally not.

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