[gdal-dev] Dealing with "default" TOWGS84 values

[Greg Troxel]

Even Rouault <even.rouault at spatialys.com> writes:

> I wanted to get opinions on a topic related to TOWGS84 7-parameter transforms. In the GDAL 
> 1.x/2.x & PROJ 4.x/5.x era, having TOWGS84 values attached to CRS definitions was critical to 
> get correct datum transformation.

> In GDAL 3, when such file is read, a BoundCRS object is returned, that is a CRS that has a base 
> CRS (potentially with a EPSG code attached. e.g EPSG:27700) + the definition of its 
> transformation to WGS84 (which corresponds to the transformation with larger area of use). 
> Software like QGIS will then use this BoundCRS (since that's what GDAL returns for the 
> dataset CRS) when reprojecting to a target CRS, which will cause the TOWGS84 parameters 
> from the file to be used (PROJ >= 6 will honour the TOWGS84 parameters of a BoundCRS 
> when transforming from/into it), rather than potentially more accurate transformations 
> (typically grid based transformations) from the base CRS to the target CRS.

It seems the crucial point is whether one intends

  to refer to a particular CRS, by just the codepoint, and therefore use
  the most precise transform, following modern best practices, or

  to refer sort of to a particular CRS, but with an inline definition of
  it (the towgs84 params) to be used instead of the modern normal way

That second method is not even what I think was ever really intended by
most, but an artifact of how it was intended to be.

So I would be inclined to ignore the towgs84 parameters when reading a
CRS, if the codepoint is in the current database, unless an option has
been given stating they should be used.

I can see what you are saying about insisting that the towgs84 values
are the standard ones in order to ignore them, but it also seems like a
file with a crs codepoint and other towgs84 values is broken.   I would
think it's better to allow it to be edited to change the CRS reference
to note that it's custom, since such a file isn't really using the
stated CRS.

Matching and skipping also feels fragile, rather than not using and
having people that want to use them have to be explicit.