[PROJ] WGS84 to GDA94 and GDA2020

[Kristian Evers]

It doesn't surprise me that you have difficulties getting a better
transformation between your WGS84-based custom CRS and the GDA94/GDA2020
CRS's. WGS84 is rather ill-defined, especially when using deprecated
constructs such as "+datum=WGS84". If your data came from a RTK GNSS
receiver I would expect it to already be in either GDA94 or GDA2020.

It is likely that you need to construct a custom transformation pipeline to
achieve your goal. You can use projinfo to get more insight to the various
transformations you are getting from cs2cs.

/Kristian

On Fri, 11 Oct 2024 at 12:27, Nigel Stewart <nigel.stewart at emesent.io>
wrote:

> > First of all I think you are being way too unrealistic about what you
> can use ChatGPT for.
> >If you'd tried the command it's suggested to you, you would know that it
> doesn't work.
> > It has given you an ill-formed CRS, as well as suggesting you use
> deprecated syntax.
>
> It didn't smell good, I must admit.
>
> > It would be helpful if you can be more explicit about what you are
> trying to do.
> > What EPSG-codes are you trying to get a transformation between?
>
> In a nutshell:
>
> # to GDA94 / MGA zone 56
>
> $ echo "0 0 0" | cs2cs +proj=tmerc +lat_0=-34.0 +lon_0=150.0 +k=1 +x_0=0
> +y_0=0 +datum=WGS84 +units=m +no_defs +type=crs +to 'EPSG:28356'
> 222908.70       6233785.28 0.00
>
> # to GDA2020 / MGA zone 56
>
> $ echo "0 0 0" | cs2cs +proj=tmerc +lat_0=-34.0 +lon_0=150.0 +k=1 +x_0=0
> +y_0=0 +datum=WGS84 +units=m +no_defs +type=crs +to 'EPSG:7856'
> 222908.70       6233785.28 0.00
>
> # to GDA2020 / MGA zone 56
>
> $ echo "222908.70       6233785.28 0.00" | cs2cs 'EPSG:28356' +to
> 'EPSG:7856'
> 222909.17       6233786.71 0.00
>
> Our expectation is that GDA2020 is within a meter or two of GDA94 due to
> the tectonic drift of the Australian plate towards the north east.
> Our concern is that from these custom transverse mercator projections,
> this difference isn't reflected.
> We can see the Helmert transformation being applied in the third case, but
> not in the first two.
> ChatGPT doesn't seem completely off track pointing to EPSG:8048 for GDA94
> to GDA2020 Helmert.
>
> Our application is RTK-based georeferencing, possibly WGS84 is not
> appropriate for our application.
>
> $ cs2cs
> Rel. 6.3.1, February 10th, 2020
>
> (But we're seeing similar for our stack using PROJ 9.4.1)
>
> Thanks in advance,
>
> - Nigel Stewart
>
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