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<p>Javier,</p>
<p>you might perhaps find some hints at
<a class="moz-txt-link-freetext" href="https://www.ga.gov.au/scientific-topics/positioning-navigation/geodesy/transform-convert">https://www.ga.gov.au/scientific-topics/positioning-navigation/geodesy/transform-convert</a>
or
<a class="moz-txt-link-freetext" href="https://www.icsm.gov.au/sites/default/files/2021-10/GDA2020%20Technical%20Manual%20V1.7.pdf">https://www.icsm.gov.au/sites/default/files/2021-10/GDA2020%20Technical%20Manual%20V1.7.pdf</a>
. There are also a number of (closed) issues on PROJ tracker about
that, because that confuses everybody, but I believe the answer is
that all and none of the 3 transformations is correct, because
there is no universal answer. Mostly comes from the fact that
GDA94 & GDA2020 are plate-anchored and WGS 84 (as EPSG:4326)
is a global datum ensemble with an accuracy of 2 m.</p>
<p>There are time-dependent transformations between GDA94 and WGS 84
(G1762), and GDA2020 and WGS 84 (G1762), but for generic WGS 84,
things are more fuzzy, as one might consider that GDA94 = WGS 84
(at 1994.0) and GDA2020 = WGS 84 (at 2020.0). <br>
</p>
<p>So when you do WGS84 to GDA2020, if you are talking about
coordinates with an epoch of ~ 2020, then the null Helmert
transformation is appropriate, since by definition GDA2020 =
ITRF2014 at 2020.0 ~= WGS84 at 2020</p>
<p>But if you are considering WGS84 coordinates with an epoch of ~
1994.0, then WGS84 ~= GDA94, and it makes sense then to propose
the transformations between GDA94 and GDA2020, hence (3) and (4).
As far as I remember the "standard" conformal transformation
between GDA94 and GDA2020 is the (non-null) 7-Helmert, and a NTv2
grid (GDA94_GDA2020_conformal) was just derived from this
7-Helmert transformation because some software prefer grids to
Helmert transformation. The proposed WGS 84 to GDA2020 (4)
actually uses GDA94_GDA2020_conformal_and_distortion, which is a
variation of the conformal grid that takes into account some
distortion (cf paragraph 3.2.2 of the GDA2020 technical manual)</p>
<p>I suspect the 3m accuracy was given so that none of the 3
transformations particularly appear as better as the others, as
ultimately the user needs to decide what "type of WGS 84" he has.<br>
</p>
<p>Just my non-authoritative guesses.<br>
</p>
<p>Even<br>
</p>
<div class="moz-cite-prefix">Le 09/05/2024 à 12:47, Javier Jimenez
Shaw via PROJ a écrit :<br>
</div>
<blockquote type="cite"
cite="mid:CADRrdKtvxK-0db457Dk0r_KHspspH3pins+JNPpP2A+R-5smMA@mail.gmail.com">
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<div>Hi</div>
<div><br>
</div>
<div>Preparing a talk for FOSS4G Europe in Tartu (see you there
if you come!), I found that the 3 transformations in EPSG
from WGS 84 to GDA2020 (2, 3 and 4) they all have 3m accuracy.</div>
<div>That is not giving a clue to PROJ about which one is
better.</div>
<div><br>
</div>
<div>(2) is a null Helmert transformation.</div>
<div>(3) is a Helmert with 7 numbers</div>
<div>(4) uses a grid.</div>
<div><br>
</div>
<div>Is there anybody from the Australian agency here? Is there
any reason why the three transformations have the same
accuracy? I would expect different values. I looks like a
copy-paste issue.<br>
</div>
<div><br>
</div>
<div>The same happens with the transformation from WGS 84 to
GDA94.<br>
</div>
<div><br>
</div>
<div>Thanks,</div>
<div>Javier<br>
</div>
<div><br>
</div>
<div>
<div>
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<div>.___ ._ ..._ .. . ._. .___ .. __ . _. . __.. ...
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