[PROJ] Vertical Transformations?

[Even Rouault]

On mercredi 20 février 2019 13:25:07 CET Greg Troxel wrote:
> Even Rouault <even.rouault at spatialys.com> writes:
> > We could enhance the PROJ database with a table/column to flag required
> > transformation, but it is probably not trivial (area of use comes into the
> > equation), and that would put the PROJ project in a position of being an
> > "authority" to decide which transformation are required or not.
> 
> That sounds like a good path.
> 
> I don't see the authority problem; when implementing a transformation
> that is defined to use a grid file, it's pretty clear that it can't be
> done the same way without it.

For horizontal datum transformations, EPSG has generally both grid-based and 
Helmert-based transformations. Which are both declared as valid. Generally the 
grid-based one will have a better declared accuracy, so might be prefered. But 
that doesn't invalidate the Helmert-based one. Failing if the grid is not 
available would be a PROJ specific policy, and not always desirable.

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