[PROJ] geoid model + Helmert

[Lesparre, Jochem]

Greg Troxel wrote:
> A) Use the geoid model to get LHN95 height from the ETRS89 HAE, based on ETRS89 horizontal.
> B) Take the original ETRS89 LLh and Helmert transform to CH1903+, and then discard the h, and swap in the LHN95 H from step A.

> What I'm having trouble with is that for a station, I expect ETRS89 HAE and CH1903+ HAE to be different, and that therefore one has to do the
> A/B procedure above.

This is indeed the correct method, but this is not really possible with a single pipeline in PROJ. If two pipelines is not an option, one workaround is temporarily storing the height in the epoch field v_4. I don't like that, because we will publish a time-dependent transformation too.

 > I don't understand how that relates to "Undoing" the grid shift. Perhaps these are two ways of expressing the same thing and that's what the push/pop is all about.

This is another workaround, that we use. From global/regional to national/local datum (from bottom to top in the pipeline below): first apply the geoid model; save the height; undo the geoid model; apply the Helmert transformation; restore the saved height.

>From national/local to global/regional datum:
+proj=pipeline
+step +proj=push +v_3
+step +proj=cart +ellps=intl
+step +proj=helmert +x=... +y=... +z=... +rx=... +ry=... +rz=... +s=... +convention=coordinate_frame +exact
+step +proj=cart +inv +ellps=GRS80
+step +proj=vgridshift +grids=geoid.tif +omit_fwd
+step +proj=pop +v_3
+step +proj=vgridshift +inv +grids=geoid.tif

I hope this clarifies what is meant with undoing the vgridshift,
Jochem



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