[PROJ] WGS 84 to GDA2020 in EPSG

[Even Rouault]

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
>
> .___ ._ ..._ .. . ._.  .___ .. __ . _. . __..  ... .... ._ .__
>
> _______________________________________________
> PROJ mailing list
> PROJ at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/proj

-- 
http://www.spatialys.com
My software is free, but my time generally not.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/proj/attachments/20240509/d1b18b8a/attachment.htm>