[PROJ] Helmert 2D

[Javier Jimenez Shaw]

Hi

Today I discovered that we have the Helmert 2D version. (Before I only used
the 3D version with geocentric coordinates).
https://proj.org/en/stable/operations/transformations/helmert.html#d-helmert

Does it make sense to use it with projected coordinates? (not geocentric,
but already projected, like UTM) Currently I am using an "affine
transformation" for a rotation, translation and scale. But for the 2D case,
the notation of Helmert is much nicer (you can see the scale and angle in
the pipeline!)

I deduce from the code that choosing 2D or 3D is based on presence of the
"theta" parameter. That also defines if the scale is in ppm or "per 1".
That means that if there is no rotation, I have to set "theta=0" if I want
the scale to be "per 1" (aka unitless), right?

Thanks.
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