Re: "Trustworthiness" of vertical transformations

Nyall,

You are indeed correct in assuming that the world of vertical
transformations isn't always straight forward. I think the best
tip I can give you is to consult the accuracy of the transformation
as reported by PROJ. In the example Even has used in the issue
that you link to it is 0.35 m. In other cases it will be "unknown accuracy"
if there is not enough information available about a given transformation.
Ultimately the accuracies are reported by the local geodetic authorities
so in general I would just trust those.

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 :-))

/Kristian

-----Original Message-----
From: PROJ <[email protected]> On Behalf Of Nyall Dawson
Sent: 26. november 2019 01:27
To: PROJ <[email protected]>
Subject: [PROJ] "Trustworthiness" of vertical transformations

Hi list,

I've seen a lot of activity on the PROJ repo lately relating to
vertical datum transformations. Coming from zero knowledge about the
world of vertical transformations... if I setup a simple
transformation between two proj CRS objects created using auth/id
codes, and transform a 3d point using proj_trans_generic... how
"trustworthy" is the resultant z value?

I've seen some examples (e.g in
https://github.com/OSGeo/PROJ/issues/1743) which use a
"EPSG:26911+5703" type construct, which leads me to believe that the
world of vertical transformations is not so straightforward! Any tips
to give a newbie entering this world to avoid the inevitable pitfalls
which await?

Cheers,
Nyall
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