[PROJ] question regarding vertical shift

[Javier Jimenez Shaw]

Hi Mathieu

I do not know if I understand your question correctly. In Switzerland the
geoid is above the ellipsoid (about 50 meters). That is a positive geoid
undulation. So the orthometric height (over the geoid) is smaller than the
ellipsoidal height.

This may clarify it:
https://www.ngs.noaa.gov/GEOID/GSVS/

Cheers
.___ ._ ..._ .. . ._.  .___ .. __ . _. . __..  ... .... ._ .__
Entre dos pensamientos racionales
hay infinitos pensamientos irracionales.



On Tue, 10 Nov 2020 at 10:39, Mathieu Pellerin <nirvn.asia at gmail.com> wrote:

> 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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