[PROJ] Serious confusion regarding towgs or not...!

[Even Rouault]

On lundi 16 décembre 2019 16:36:52 CET Nyall Dawson wrote:
> Hi list,
> 
> Been struggling through fixing some QGIS proj 6 related issues today,
> and I've got to admit - I'm very confused about the role of towgs84 in
> the proj 6 world.
> 
> My current crop of questions:
> 
> 1. Is towgs84 effectively deprecated? Should I be avoiding it as much
> as possible? Is it inherently evil?

It is not "evil", but the world of coordinate transformations is full of traps 
and headaches.
It associates a transformation (among several potential ones) to WGS84 to a 
CRS. Once it is set, PROJ will honour it and will not try potentially better 
transformations that may use grids. So "+proj=longlat +ellps=GRS80 
+towgs84=,0,0,0" is a potential way of encoding boths GDA94 and GDA2020, but 
if you use it, then you'll get a null transformation between both.

> 2. When I identify various WKT definitions using projinfo, I get a mix
> between direct matches and matches to "BoundCRS of ...".

I should warn that identification is a divination exercice / best effort / 
fuzzy logic, not something meant to be fully bullet proof.

> I'm unsure
> when this BoundCRS is important or not. Should I be treating a CRS
> identified as a BoundCRS of ... as identical to the identified CRS,
> and is it safe to just use the identified auth/code combo for that
> CRS? Or is that losing important information and I should NEVER treat
> a BoundCRS identified match as being an identified result in any
> circumstance?

I don't think there's an absolute answer. Depends on the context and the 
origin of that WKT. If it is a legacy WKT from GDAL < 3 & PROJ < 6 world, then 
you might just keep the Source CRS of the BoundCRS as the most significant 
part, and consider that the transformation to WGS84 captured in the 
ABRIDGEDTRANSFORMATION was not necessarily intended to be the one used.

I see a number of tickets in PROJ tracker are files about several issues 
related to that. I'm not sure we'll ever manage to find the answer that 
satisfies both practicality and "correctness".

Even

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