[PROJ] WGS84 to GDA94 and GDA2020

[Nigel Stewart]

> 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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