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

Sean Gillies sean at mapbox.com
Wed Jun 2 12:56:07 PDT 2021


Sounds good. Until there is consensus on what coordinate epoch means for
OGC:CRS84 GeoJSON, the official and most widely used kind, I think it would
be better if GDAL didn't extend the format. For now, applications that need
more precision can and should use another format.

On Thu, May 27, 2021 at 12:19 PM Even Rouault <even.rouault at spatialys.com>

> 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.
> Even
> 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>
> 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) 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) 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>
>> wrote:
>> >>
>> >> Can I make the suggestion that a subset of
>> >> 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
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