[PROJ] Precedence of multiple vertical grids

[Lesparre, Jochem]

Hi,

After the PROJ. tutorial at the EUREF symposium I got started with PROJ.6. I'm very happy, as this solves many of my problems with introducing a new national transformation for the Netherlands. However, I do notice some odd behaviour now and then. This is the first one.

The order of precedence of vertical grids in PROJ. seems not to work as described in the documentation (https://proj.org/operations/transformations/vgridshift.html?highlight=gridshift):

(base) D:\>cct +proj=pipeline +step +proj=vgridshift +grids=D:\nlgeo2018beta.gtx,null
5 52 43
  5.0000000000   52.0000000000       43.0000           inf
0 0 43
  0.0000000000    0.0000000000       43.0000           inf
^Z

(base) D:\>cct +proj=pipeline +step +proj=vgridshift +grids=null,D:\nlgeo2018beta.gtx
5 52 43
  5.0000000000   52.0000000000       -0.4754           inf
0 0 43
  0.0000000000    0.0000000000       43.0000           inf
^Z

Confusingly, for horizontal grid shifts PROJ. does work as described in the documentation:

(base) D:\>cct +proj=pipeline +step +proj=hgridshift +grids=D:\rdtrans2018beta.gsb,null
5 52 43
  4.9996207144   51.9990307906       43.0000           inf
0 0 43
  0.0000000000    0.0000000000       43.0000           inf
^Z

(base) D:\>cct +proj=pipeline +step +proj=hgridshift +grids=null,D:\rdtrans2018beta.gsb
5 52 43
  5.0000000000   52.0000000000       43.0000           inf
0 0 43
  0.0000000000    0.0000000000       43.0000           inf
^Z

I tend to say that the documentation and the horizontal grid shift are correct and that the code for the vertical grid shift should be adapted?

Kind regards, Jochem



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