[PROJ] Help understanding grid files and +towgs84

[Nathan Woodrow]

Hi,

So I'm mainly just trying to understand how and why this works just for my
own knowledge.

I'm currently working on a GDA94 > GDA2020 transform project and noticed
that the proj4 string that ships with everything (GDAL, QGIS, PostGIS) only
has +proj=utm +zone=55 +south +ellps=GRS80 +units=m +no_defs but in order
to get the transform to work correctly I need to add the grid to the source
and the +towgs84=0,0,0,0,0,0,0 to the target (GDA2020) proj4 sring.

ogr2ogr command was:

ogr2ogr -s_srs "+proj=utm +zone=55 +south +ellps=GRS80
+towgs84=0,0,0,0,0,0,0 +units=m +no_defs +wktext
 +nadgrids=C:\temp\GDA94_GDA2020_conformal.gsb"
-t_srs "+proj=utm +zone=55 +south +ellps=GRS80 +towgs84=0,0,0,0,0,0,0
+units=m +no_defs +wktext" out.shp in.shp

Now I'm just trying to understand mainly:

1) Why is +towgs84=0,0,0,0,0,0,0 needed on the target? As if it's not there
it will just return the same values as the input
2) Why is that not included as default?
3) Is my understanding of the following correct?  Proj is really doing:
GDA94 -> WGS84 -> Shifted WGS84 via grid -> GDA2020?

Thanks,
Nathan
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