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

[Even Rouault]

> Hm. I'm now considering just dropping all code from QGIS which relates
> to handling towgs and BoundCRS, and leaving the responsibility
> entirely with GDAL and PROJ to insert or ignore TOWGS wherever it's
> desirable/wanted.
> 
> From a QGIS perspective I think the biggest downside of doing this
> would be that many CRSes would be shown as "custom" user crses,
> instead of via an auth/code identifier. But the alternative is that we
> end up with a lot of logic relating to boundcrs downstream, and that
> makes me nervous. 

Ah ah, that you rely entirely on PROJ to do always the "right thing" makes me 
equally nervous :-)
Perhaps QGIS when ingesting a WKT1 string with a TOWGS84[] or a PROJ.4 string 
with a +towgs84 should ask the user: "do you really want to use that precise 
transformation to WGS84 (which will thus make it the pivot CRS for datum 
transformations) or just use the underlying CRS and being proposed with 
several possibilities when transforming to other datums ?" Of course most 
users will have no clue at answering that question... Sometimes users 
implictly relied on the more or less arbitrary +towgs84 values that were 
picked up previously, and they will suffer from misalignment if using other 
transformations. So perhaps there should be a different behaviour if opening 
old QGIS projects than if creating new ones ? Just brainstorming

Even

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