[PROJ] How to use the tag GEOIDMODEL

[Javier Jimenez Shaw]

Hi

When there are several geoid models for a vertical system, is there a way
to force in WKT2 to use a specific one?
I know in the US I can use the values in the geoid_model table (for
instance using the method AuthorityFactory::getGeoidModels), GEOID18 or
GEOID12B. Or GGM10 for Mexico, that also uses NAVD88.
        GEOIDMODEL["GEOID18"]

However there are other places where those geoid models are not added to
the proj.db database, like
fr_ign_RAF18.tif: France, IGN France, RGF93 (EPSG:4965) to NGF-IGN69 height
(EPSG:5720)
and
fr_ign_RAF09.tif: France, IGN France, RGF93 (EPSG:4965) to NGF-IGN69 height
(EPSG:5720)

Even Rouault mentioned some time ago that the epsg.org purpose of that
field was to set not just a name, but the transformation id itself, as
shown in http://docs.opengeospatial.org/is/18-010r7/18-010r7.html , section
10.4
        GEOIDMODEL["CGG2013",ID["EPSG",6648]]

So to force the French vertical crs to use one or the other we should write
something like
        GEOIDMODEL["RAF09",ID["EPSG",8371]]
or
        GEOIDMODEL["RAF18",ID["EPSG",8885]]

Is that right? Is that working in PROJ 9.1.0? I tried in cs2cs, but the
output was identical. The log showed that both were using raf18 tif file.
Could it have many entries to choose one automatically? I see in the table
geoid_model that there are several "operation_code" for the same "name",
and PROJ chooses one.

I was looking the code trying to understand how the tag GEOIDMODEL is used
in a transformation, but didn't get anything.

Thanks.

PS I do not know how many vertical systems can use different geoid models,
nor what may happen in the future when some countries update their geoid
models.
.___ ._ ..._ .. . ._.  .___ .. __ . _. . __..  ... .... ._ .__
Entre dos pensamientos racionales
hay infinitos pensamientos irracionales.
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