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<p>> <span style="white-space: pre-wrap">So, I agree with you that using the reference epoch, like PROJ does, is the most reasonable thing to do for the moment.</span></p>
<p><span style="white-space: pre-wrap">Just to clarify that if you use a time-dependent Helmert transformation without specifying an epoch, PROJ will only apply the non-time-dependent terms. So the "reference epoch" in that situation is the one of the transformation, not the frame reference epoch of the datum</span></p>
<p><span style="white-space: pre-wrap">Cf:
</span></p>
<p><span style="white-space: pre-wrap">$ echo 1 0 0 | bin/cct +proj=helmert +x=1 +dx=.1 +t_epoch=1234
2.0000 0.0000 0.0000 inf
</span></p>
<p><span style="white-space: pre-wrap">$ echo 1 0 0 1235 | bin/cct +proj=helmert +x=1 +dx=.1 +t_epoch=</span><span
style="white-space: pre-wrap">1234</span><br>
<span style="white-space: pre-wrap"> 2.1000 0.0000 0.0000 1235.0000</span><span
style="white-space: pre-wrap">
</span></p>
<p><span style="white-space: pre-wrap">
</span></p>
<p><span style="white-space: pre-wrap">PROJ 9.5 has now a new API function, proj_coordoperation_requires_per_coordinate_input_time(), to indicate you if a given coordinate operation expects time-dependent terms: <a class="moz-txt-link-freetext" href="https://github.com/OSGeo/PROJ/pull/4221">https://github.com/OSGeo/PROJ/pull/4221</a> .</span></p>
<p><span style="white-space: pre-wrap">There's also an explicit error code for the transformations where the coordinate epoch is really required, so currently +proj=defmodel and +proj=deformation +t_epoch= . For time-dependent Helmert, this is considered optional as mentioned above (mostly for backward compatibility)</span></p>
<p><span style="white-space: pre-wrap">Even
</span></p>
<p><span style="white-space: pre-wrap">
</span></p>
<pre class="moz-signature" cols="72">--
<a class="moz-txt-link-freetext" href="http://www.spatialys.com">http://www.spatialys.com</a>
My software is free, but my time generally not.</pre>
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