[PROJ] question regarding vertical shift

[Mathieu Pellerin]

Javier, useful link, I think it did provide me with some higher level of
clarity :)

Thanks,

Mathieu

On Tue, Nov 10, 2020 at 5:10 PM Javier Jimenez Shaw <j1 at jimenezshaw.com>
wrote:

> 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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>>
>
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