[PROJ] EPSG v10 update status

[Nyall Dawson]

On Mon, 12 Oct 2020 at 03:16, Even Rouault <even.rouault at spatialys.com> wrote:
>
> > How would you imagine a downstream project like QGIS should respond to
>
> > this concept? Should we add a user-visible flag on any CRS definitions
>
> > associated with a datum ensemble to say "Warning: ensemble datum, not
>
> > for use in accurate spatial referencing"? What other user-facing
>
> > changes do you think should be introduced?
>
>
>
> That's a tough question. Depends if we want to stress users or not :-)
>
>
>
> For example, when you open a dataset referenced to WGS84, I don't think we should put a warning. The user isn't responsible for the choice of the data producer.

True, but they ARE responsible for correctly interpreting the accuracy
of the inputs to their analysis. I really think we need to be
aggressive about our messaging that EPSG:4326 isn't the "magical
golden ticket single answer" to the question "what CRS should I use
for my data/analysis". It's such an ingrained  default choice for the
vast majority of spatial data users, and so many of these users don't
even realise that there's any issue here.

> With PROJ 8 / epsg10_part2, the WKT:2019 output of a CRS using WGS84 will show the ENSEMBLE[] and the 2m ensemble accuracy, so that's already a form of warning (but probably most people not aware of the issue won't realize what this means)

Is the 2m accuracy for the ensemble correct here? I was of the
understanding that the inherent ambiguity in EPSG:4326 is in the order
of 10s of meters!

> When a user saves a layer from a CRS that doesn't use a datumensemble to another one that uses one, perhaps a warning could be issued. But I'm not sure if we should do that. That also depends on the accuracy of the data itself. Reprojecting something with a 100m accuracy to WGS84 isn't really a problem. There are so many ways in which users can shoot themselves in the foot. Almost anything involving datum transformation introduces extra inaccuracy.

Right -- but again, I think our responsiblity would be to just show a
warning to users so that they have ALL the information available to
make informed choices. The way I see it:

- no warning:
    - low accuracy data: no issue
    - high accuracy data: data is silently degraded and only informed
users will know that this has occurred
- with warning:
   - low accuracy data: a spurious "noisy" warning, annoys users.
(could potentially be avoided with a "[  ] Don't show this again"
option on the warning.)
   - high accuracy data: all users are informed that there's an issue,
they've got the choice to ignore the issue and degrade their data or
make a better choice and not taint their data

> There's also the fact that there are none projected CRS available in EPSG based on any of the realizations of WGS84 or ETRS89... So users have no easy choice of a better alternative, if they want to stay in the 'family' of the datum ensemble.

Still, that's THEIR choice. At least they've been made aware of the
consequences of their decision...

> And not all datum ensembles are the same. (well, there are just 2 geodetic datums in EPSG for now :-)). But the ensemble accuracy of ETRS89 is 0.1 m. Not the same story as the 2 m of WGS84. So for European users ETRS89 is quite a reasonable choice for a lot of use cases. I don't believe the use of a given realization of it is very frequent, at least in the GIS field. People would actually rather use a national datum when available (in France RGF93 which in its latest version is a realization of ETRF2000 at epoch 2009.0, similar story for CHTRF95 in Switzerland, etc.), mostly for legal reasons, than a generic pan-european ETRF realization.

So maybe we should include the ensemble accuracy in the warning. Something like

"Warning: saving this dataset to the xxxxx ensemble datum used by
EPSG:xxxx will result in a maximum positional accuracy of x.x meters.
If higher accuracy is required then an alternative CRS should be used.
(Please contact even.rouault at spatialys.com to ask which CRS he'd
recommend using for your particular circumstances.)" ;)

Nyall





>
>
>
> That's just my current thoughts. I've no definitive opinion. Looks to me datum ensemble are more a band aid to reflect that the current situation is not ideal, but the ideal has not been drawn (yet?)
>
>
>
> Even
>
>
>
> --
>
> Spatialys - Geospatial professional services
>
> http://www.spatialys.com