[PROJ] Pipeline: Cartesian transform to change from ellipsoid to sphere

[Even Rouault]

Le 12/08/2024 à 18:44, Laura, Jason R via PROJ a écrit :
> List,
>
> I have a set of body-centered body-fixed coordinates that were 
> computed with an ellipsoid of +a=3396190 +b=3376200. I am attempting 
> to reproject these into a spherical datum (+R=3396190) using the 
> pipeline syntax. For example,
>
>         +proj=pipeline +units=m
>         +step +proj=cart +a=3396190 +b=3376200
>         +step +proj=cart +R=3396190
>
> Which is failing with a unit warning. I suspect I am missing something 
> basic.
You need to add +inv in the last step to go from ECEF to lon-lat 
coordinate space
>
> I have looked at the geodesic -> Helmert -> geodesic transformation 
> tutorial, but I am clearly misunderstanding something 
> (https://proj.org/en/9.3/tutorials/EUREF2019/exercises/pipelines.html#geodetic-helmert-geodetic)
>
> Looking at these docs, I tried:
>
>         +proj=pipeline
>         +step +proj=cart +a=3396190 +b=3376200
>         +step +proj=helmert +z=19990 +convention=position_vector
>         +step +inv +proj=cart +R=3396190
>
> But I get an invalid latitude error (which I believe means I am 
> misspecifying the input system).
Works for me. The input coordinate order is longitude, latitude, Z.
>
> Any suggestions appreciated,
> Jay
>
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