[PROJ] Transforming from epsg:4326 to epsg:31468 produces a 2 meter offset

[Alan Snow]

Original question: https://github.com/pyproj4/pyproj/discussions/814

What he is asking is if TOWGS84 parameters are needed for datum
transformations.

Thanks 👍

On Thu, Apr 15, 2021, 2:32 AM XCTrails <info at xctrails.org> wrote:

> Hi,
>
> I have input polygon in epsg:31468 which I need to transform to epsg:4326
>
> My current code boils down to this:
>
> project = pyproj.Transformer.from_crs("EPSG:31468","EPSG:4326",
> always_xy=True)
> poly = transform(project.transform, poly)
>
> The problem however is that the resulting polygons have a 1-2 meter offset
> according to visual inspection in QGIS. On reddit, someone suggested that I
> might also need a datum transformation, on the pyproj4 issue tracker
> someone said, that datum transformations are no longer needed in PROJ 6+
> and that I should ask for clarification here. So here I am ;-)
>
> Can anyone shed any light on why I get these offsets and what I can do to
> fix this?
> There's no offset if I manually reproject this in QGIS and compare it with
> the original.
>
> Thanks,
> Armin
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