[PROJ] question regarding vertical shift

[Mathieu Pellerin]

Greetings,

I’m hoping the mailing list can enlighten me regarding handling of vertical
shift in PROJ.

Using this Switzerland vertical shift grid (
https://www.dropbox.com/s/6uzpg3gr47q1p1k/CHGeo04.gtx?dl=0), if I type in
the following command to adjust vertical height (100 meters here):

echo 7.9995 46.7949 100 | cct +proj=vgridshift +grids=CHGeo04.gtx

PROJ returns the following result:

7.9995000000   46.7949000000       49.8406           inf

The adjusted height is 49.8 meters. This puzzles me as the grid’s value at
the provided longitude and latitude (i.e.  7.9995 46.7949) is 50.179. I
would have assumed that this meant a vertical adjustment of +50.179, not
-50.179 as observed by the PROJ returned values.

Is PROJ’s result the correct one based on the grid? Or should an inverse
transformation (i.e. cct -t) be used here? If so, what’s the rationale of
whether to pick forward or inverse transformation?

Regards,

Mathieu Pellerin
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