[PROJ] "Fallback" support for transformations

[Even Rouault]

Hi Nyall,

As in QGIS not all coordinate transformations need a geodetic-class level of 
accuracy, one aspect you didn't mention and that should probably be taken into 
account is the desired final accuracy, which can be linked with the current 
map scale.

One factor to also consider is the amount of shift introduced by a 
transformation. But this information is not easily available. This is, I 
believe, at worse 200 meters when transforming from old systems to new ones, 
and much smaller for transformations between more recent systems (at least for 
Earth based transformation... No idea for potential future non-Earth 
transformations ...)

Doing a quick computation, at 1/1,000,000 scale, and 96 DPI screen resolution, 
a pixel represents 1e6 / (96 / 0.0254) = 264 m. So for scales equal or smaller 
to that, the effect of any datum transformation is invisible at screen.

Now, if for a given (A,B) tuple of source and target CRS, you've a grid-based 
transformation and a Helmert one, then the difference in results between both 
is probably at worse of the order of 5 m (and often < 1 m).
So the difference between both would only start being visible for scales equal 
or larger than 1/25,000 ( 25000 / (96 / 0.0254) = 6.6 m)

That would mean that, when doing a coordinate transformation in QGIS, if you 
had a hint of the scale for which it is intended, then you could decide if you 
can just blindly use proj_create_crs_to_crs_from_pj() or not.

(Regarding what Jochem describes for the Dutch transformation, the variant 
with the fading out is indeed a smart approach, but too smart to be encoded in 
the EPSG database (at least currently), and thus being automatically proposed 
by PROJ in a proj_create_crs_to_crs_from_pj() / proj_create_operations() use 
case. Could probably be done with a custom entry in the PROJ db, but that's 
another topic. So for now, you need to specify "at hand" such PROJ pipeline. )

Even

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