[PROJ] Local CRS as derived

[Javier Jimenez Shaw]

Hi

I want to define a local projected CRS, and be able to compute
transformations from geographic 3D coordinates to this final system (site
calibration). This final (3 axes) system have to be expressed in a WKT2
string, including somehow the coordinate operations.

It looks like a DERIVEDPROJCRS is a good candidate (I got inspired in
https://gis.stackexchange.com/questions/387189/how-to-use-affine-transform-parameters-in-a-proj-or-wkt2-string
). Maybe there is something better.
I would choose the BASEPROJCRS properly (something like a Transverse
Mercator centred in the area of interest), and I can compute the 2D affine
transformation parameters.

However I have some questions with the vertical component. I guess there is
not affine transformation method in 3D ("Affine parametric transformation"
is 2D), so I would have to define a compound of "derived projected" and
"derived vertical", right?. For the vertical I could use the method
"Vertical offset" (is there any other option that is not just a constant
value?). But that means that I need a proper vertical crs as base. If the
initial 3D input is in ellipsoidal heights (very likely, as they are RTK
measurements), how can a add a derived vertical to change the Z value?
There is no such a vertical crs that means "ellipsoidal height", even in
WKT2, right?

This solution would leave me only 5 degrees of freedom: 3 for the
translation, 1 for the horizontal rotation and 1 for horizontal the scale
(now I notice that I would not have vertical scale). Vertical and
horizontal calibrations would be independent.

How can I deal with the vertical component?
Is there any better solution?

Thank you

Javier.
.___ ._ ..._ .. . ._.  .___ .. __ . _. . __..  ... .... ._ .__
Entre dos pensamientos racionales
hay infinitos pensamientos irracionales.
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