[PROJ] How to use the tag GEOIDMODEL

[Even Rouault]

Javier,
>
> 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.

Yes, currently that will not work

The relevant code is in 
src/iso19111/operation/coordinateoperationfactory.cpp at the bottom of 
createOperationsGeogToVertFromGeoid() method.

         const auto &models = vertDst->geoidModel();
         for (const auto &model : models) {
             const auto &modelName = model->nameStr();
             const auto transformations =
                 [...]
                     : authFactory->getTransformationsForGeoid(
                           modelName,
context.context->getUsePROJAlternativeGridNames());

So it uses the AuthorityFactory::getTransformationsForGeoid() method, 
which queries the geoid_model database table, to find which operations 
are associated with a geoid model name.
When this returns an empty set, createOperationsGeogToVertFromGeoid() 
should probably fallback to querying for a transformation by name using 
modelName (assuming official names like "RGF93 v2 to NGF-IGN69 height 
(2)" are used), and probably start by instantiating from the 
identifier(s) (returned by model->identifiers()) (EPSG:8371) associated 
with the model object when it is avaiable. Both can be done with 
respectively the createObjectsFromName() and createCoordinateOperation() 
methods of AuthorityFactory.

> 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.

Yes, PROJ will apply its usual logic (don't ask me to explain in detail 
what it is!, or rather use 
https://proj.org/operations/operations_computation.html as a first order 
approximation of what it does) to select /sort when there are several 
operation candidates.

Even


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