[PROJ] WGS 84 to GDA2020 in EPSG

[Javier Jimenez Shaw]

Thanks Even. That content would fit perfectly in the talk... but it is only
20 min. I will see how much time I have to go into it.

On Thu, 9 May 2024 at 14:40, Even Rouault <even.rouault at spatialys.com>
wrote:

> 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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