[PROJ] geoid model + Helmert

[Lesparre, Jochem]

Even Rouault wrote:
> yes, your analysis is fully correct , and your enhanced pipeline too. However that would suck a bit to have to implement it like that as
> doing and undoing the vgridshift is inefficient. The ideal solution would use some storage space as we discussed a few months ago but
> couldn't reach a conclusion how to implement that.  For 3D operations only like that, we could use the v_4 component as the temporary
> storage using push +v_3 / axisswap order=3,4 / pop +v_3 trickery

Greg Troxel wrote:
> What I think you want is to get a grid file that is indexed in CH1903+ and that gives the difference from CH1903+ HAE to LHN95 height (or
> defines LHN95, giving an estimate from CH1903+ HAE to LN02).   I don't
> understand why that isn't what's available.

Probably that isn't available because the usual way to combine a geoid model and a Helmert transformation is to base them both on a international/regional CRS (e.g. ETRS89). I think that this problem should be fixed in the PROJ code, not by asking national authorities to make redundant geoid models.

I'm working for the national authority of the Netherlands (NSGI). We find using the v_4 component as the temporary storage too confusing, when also publishing time-dependent transformations. Therefore, we publish a pipeline with the inefficient doing and undoing the vgridshift. However, I'm still hoping for a conclusion for the discussion of a few months ago on how to implement the ideal solution.

Jochem



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