[PROJ] "Trustworthiness" of vertical transformations

[Even Rouault]

On mercredi 27 novembre 2019 09:34:33 CET Nyall Dawson wrote:
> On Tue, 26 Nov 2019 at 20:52, Kristian Evers <kreve at sdfe.dk> wrote:
> > You can get the transformation accuracy using the API function
> > proj_coordoperation_get_accuracy(). I think it would be cool if
> > information about transformation accuracy where readily available in
> > software like QGIS (nudge, nudge :-))

I should mention that the information about accuracy should be taken with a  
grain of salt for several reasons:
- the EPSG guidance note 7-1 [1] underlines that the exact definition of 
accuracy varies according to geodetic agencies
- in a number of situations, PROJ will have to synthetize the resulting 
accuracy when chaining several steps. The algorithm currently is rather 
simple: just add the (in)accuracies of each step. I do have some vague 
remembering from univ that this was not always the "right" thing to do from a 
math point of view. I guess sometimes perhaps taking the max() would be more 
appropriate. But given that we can potentially mix uncomparable things, 
probably not that a big deal...

> That's where I'm coming from now... I'm just wondering if and how we
> should expose vertical transformation functionality. Do you have any
> ideas on what functionality you would expect an end-user application
> to expose for vertical transformations?

Depends on the use cases. If you want to aggregate point cloud datasets that 
are referenced against several vertical CRS into a single dataset, you will 
probably want to convert them into a single Geographic CRS (ITRF2014, WGS84 
G1762) or a single CompoundCRS appropriate for the area of study so that they 
can be stashed together.

Even

[1] http://www.epsg.org/Portals/0/373-07-1.pdf , §6.5.4.1 Coordinate Operation 
accuracy

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