Javier,
you might perhaps find some hints at
https://www.ga.gov.au/scientific-topics/positioning-navigation/geodesy/transform-convert
or
https://www.icsm.gov.au/sites/default/files/2021-10/GDA2020%20Technical%20Manual%20V1.7.pdf
. 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.
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).
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
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)
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.
Just my non-authoritative guesses.
Even
Le 09/05/2024 à 12:47, Javier Jimenez Shaw via PROJ a écrit :
Hi
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.
That is not giving a clue to PROJ about which one is better.
(2) is a null Helmert transformation.
(3) is a Helmert with 7 numbers
(4) uses a grid.
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.
The same happens with the transformation from WGS 84 to GDA94.
Thanks,
Javier
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