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Following this answer, I was able to convert from GDA94 to GDA2020 using proj string (I'm using PostGIS).

Now is there a way to convert it back? I tried to invert the signs from @JasonD answer but it didn't work:

+proj=longlat +ellps=GRS80 +towgs84=0.06155,-0.01087,-0.04019,0.0394924,0.0327221,0.0328979

When I use the above string on the converted geometry the coordinates are not the same from what they were before the conversion.

Can someone advise how to do it?

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    GDA2020 is not a parameterized transformation, it is a NTv2 grid from ICSM icsm.gov.au/gda2020 have a read of gis.stackexchange.com/questions/310567/… to use NTv2 gsb files in PostGIS. Commented Apr 22, 2020 at 1:47
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    The proj4 strings describe the projection, i.e., there is one string for GDA94 and one for GDA2020. Just exchange them.
    – CL.
    Commented Apr 22, 2020 at 10:14
  • @MichaelStimson That's not true. There are coordinate frame and NTv2 grid files to convert between GDA94 and GDA2020.
    – mkennedy
    Commented Apr 23, 2020 at 0:53
  • How much different? If you don't have ellipsoid height values, you won't exactly 'round-trip.'
    – mkennedy
    Commented Apr 23, 2020 at 1:30
  • @mkennedy while it is true that there is a 7 parameter transformation that was introduced as a stopgap it doesn't take into account the gravity distortion which gives a slight difference in the coordinates. You can do an X, Y shift without having to bounce off ellipsoidal heights using the GSB. It all depends on how accurate you need to be, if your accuracy figure is more than 5 metres you could probably get away with using the coordinates interchangeably as the difference is 1-2 m E and 1.5-3 m N in UTM, if someone checks your result using the ICSM calculator you'd be disproved though. Commented Apr 23, 2020 at 5:02

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