[PROJ] Vertical Transformations?

[Even Rouault]

On mercredi 20 février 2019 08:12:34 CET Greg Troxel wrote:
> Even Rouault <even.rouault at spatialys.com> writes:
> > With the vertconc.gtx grid available, you'll get
> > 
> > $ echo "45 -100 100" | cs2cs -f "%.8f" EPSG:7406 EPSG:5500
> > 44.99998071	-100.00039555 30.85853056
> > 
> > Without it, you'll get just us-ft to metre for the vertical part:
> > 44.99998071	-100.00039555 30.48006096
> 
> With the grids being in separate distribution files, it seems easy to
> end up with them not installed.

Yes, but as the number of grids is growing over time, putting them in a single 
package would be an annoyance for people who have local needs.

> 
> I wonder if transformations that need grid files for correct behavior
> should fail rather than skip the transformation, at least by default.

That's a complex topic. There is no "correct" behavior per-se. All 
transformations are approximations, so it is a matter of how accurate your 
needs are. In the above example if you're fine with ~ 1 metre vertical 
accuracy, you don't need the VERTCON grids.
The EPSG database can also reference some grids that can not necessarily 
easily be found, are not free, or have no known conversion to a PROJ 
recognized format.

Selection of a given transformation pipeline among all possible also involve a 
number of arbitrary decisions (some transformations might have intersecting 
area of validity). Playing with the new projinfo utility to list available 
coordinate operations can show the complexity of this...

Even

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