[PROJ] Knowing if geoid undulation is applied in C++

[Even Rouault]

Javier,

> Let me ask this in 3 levels (all in C++):
> - Given two CRS via WKT, is there any way to know if the transformation
> from one to the other is applying a geoid undulation?
> - ... if the transformation is applying any grid file transformation?

If you don't use the black-box API of proj_create_crs_to_crs() but use instead 
CoordinateOperationFactory::createOperations() and then on each result call 
CoordinateOperation::gridsNeeded(), you can get the grids that will be used. 
To know if a grid is for vertical correction, you'll have to analyze the PROJ 
pipeline string and look for +proj=vgridshift +grids=...

You can do that with the corresponding PROJ C API as well

> - ... when I run it from GDAL?

No, if you use OGRCoordinateTransformation without specifying an explicit 
pipeline, it is a black-box.

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