<div dir="ltr"><div>Even,</div><div><br></div><div>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.</div><div><br></div><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, May 27, 2021 at 12:19 PM Even Rouault <<a href="mailto:even.rouault@spatialys.com">even.rouault@spatialys.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div>
<p>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).</p>
<p>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 <span><span><em>OGC Features</em> and <em>Geometries
JSON</em> SWG </span></span> comes with something regarding
this.<br>
</p>
<p>Even<br>
</p>
<div>Le 27/05/2021 à 19:24, Sean Gillies via
gdal-dev a écrit :<br>
</div>
<blockquote type="cite">
<div dir="ltr">
<div>Hi all,</div>
<div><br>
</div>
<div>I've got a suggestion about limiting the number of formats.</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>GML, GeoPackage, PostGIS, and the actively used raster
formats like GeoTIFF seem like the important ones to me.</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Thu, May 27, 2021 at 7:40
AM Even Rouault <<a href="mailto:even.rouault@spatialys.com" target="_blank">even.rouault@spatialys.com</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi,<br>
<br>
- merging the underlying API without any format support is I
believe of <br>
little interest. So I'll wait for at least one format
(likely <br>
GeoPackage) to have merged in the master of their
specification an <br>
official way of storing the coordinate epoch. I've also
prepared an <br>
enhancement of the GeoTIFF spec regarding this <br>
(<a href="https://github.com/opengeospatial/geotiff/pull/99" rel="noreferrer" target="_blank">https://github.com/opengeospatial/geotiff/pull/99</a>)
that will likely be <br>
discussed at the next OGC GeoTIFF SWG meeting.<br>
<br>
- I've just chatted with Howard and a good compromise could
be that for <br>
formats that will have an official way of storing the
coordinate epoch, <br>
we store it by default (when it is set), and for formats
that we <br>
unilaterally extend (GML, KML, GeoJSON, etc.), we require an
explicit <br>
GDAL_ENABLE_COORD_EPOCH_STORAGE=YES configuration option to
be set <br>
(default would be NO). We might revisit in the future the
default value.<br>
<br>
- On the vector side of things, things will probably get
more <br>
complicated for some drivers, as it is likely that
Spatialite (see <br>
<a href="https://groups.google.com/g/spatialite-users/c/NtbuBpRzYBE" rel="noreferrer" target="_blank">https://groups.google.com/g/spatialite-users/c/NtbuBpRzYBE</a>)
or PostGIS <br>
might have per-geometry coordinate epoch, not at the layer
level. But I <br>
don't think that would invalidate having support for it at
the layer <br>
level (those database already support a per-geometry CRS,
but GDAL <br>
support for that is probably lacking a bit in those
drivers), as a <br>
OGRGeometry can potentially be associated with its own <br>
OGRSpatialReference object.<br>
<br>
Even<br>
<br>
Le 27/05/2021 à 15:01, Howard Butler a écrit :<br>
><br>
>> On May 26, 2021, at 8:33 PM, Nyall Dawson <<a href="mailto:nyall.dawson@gmail.com" target="_blank">nyall.dawson@gmail.com</a>>
wrote:<br>
>><br>
>> Can I make the suggestion that a subset of<br>
>> <a href="https://github.com/OSGeo/gdal/pull/3827" rel="noreferrer" target="_blank">https://github.com/OSGeo/gdal/pull/3827</a>
could be created and be merged<br>
>> on its own? Specifically the commits which add the
underlying API for<br>
>> GDAL to handle epochs should be controversy-free
and suitable for<br>
>> merge outside of the larger/trickier question of
patching in support<br>
>> to the data formats.<br>
> :thumbsup:<br>
><br>
> 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).<br>
><br>
> Howard<br>
><br>
><br>
</blockquote>
</div>
<div><br></div></div><pre></pre></blockquote></div></blockquote></div><div><br></div>-- <br><div dir="ltr" class="gmail_signature"><div dir="ltr">Sean Gillies</div></div></div>