[Qgis-user] Unexpected comparison Web-Mercator vs Mercator

[Javier Jimenez Shaw]

Hi Richard

Thanks for testing it.
I don't know if it is an issue or a feature (not kidding. It could be a
simplification to be faster, with a way to disable it somewhere in the
settings). That's why I asked here in the mailing list, not in github.
I just want to know the reason.
My workaround is to reproject with ogr2ogr.

Please, let me know the issue number when you create it. I can add there
any needed extra information.

Thanks,
Javier.

On Wed, 23 Jul 2025 at 14:44, Richard Duivenvoorde via QGIS-User <
qgis-user at lists.osgeo.org> wrote:

> Mmm, funny :-)
>
> I can reproduce this.
>
> So only with the 'Identify features' popup shown, the second line seems to
> be rendered???
>
> As if the rendering of the mapcanvas is different then the rendering of
> 'rubberband'...
>
> I would create an issue (with a small screencapture of the hovering,
> because it took me some time to figure out what you meant :-) )
>
> Regards,
>
> Richard Duivenvoorde
>
>
>
> On 22-07-2025 16:50, Javier Jimenez Shaw via QGIS-User wrote:
> > Hi
> >
> > I am trying to compare web-mercator (EPSG:3857) with Mercator
> (EPSG:3395) projections.
> > For that purpose I think that a straight line in EPSG:3395 (loxodromic)
> shouldn't be exactly straight in EPSG:3857.
> >
> > To show that, I created two linestrings in EPSG:3395, one with one
> segment, and the second with two segments co-linear (just adding a point in
> the middle). If I then select EPSG:3857 in QGIS, there should be a small
> difference... but I do not see it!
> >
> > Only if I use the "Identify features", click in the area (at the proper
> zoom level), and hover over "identify all" or "2p". Then a second line
> appears! But it disappears when I select anything. (see attached screenshot)
> >
> > The attached gpkg files are just the geometries as one segment (2p.gpkg)
> and 2 segments (3p.gpkg), both defined in EPSG:3395
> >
> > The coordinates of the line are (0,0) and (8e6, 10e6) Yes, a long line
> to make it more visible.
> > the midpoint in EPSG:3395 is (4e6, 5e6). That is easy.
> >
> > Reprojecting the two points into EPSG:3857 we have (0,0) and
> >
> > echo 8e6 10e6 | cs2cs EPSG:3395 EPSG:3857
> > 8000000.00 10039255.88 0.00
> >
> > The midpoint is
> > echo "scale=2; 8000000.00 / 2; 10039255.88 / 2" | bc
> > 4000000.00
> > 5019627.94
> >
> > The midpoint in EPSG:3395 and then projected to EPSG:3857 is this:
> > echo 4e6 5e6 | cs2cs EPSG:3395 EPSG:3857
> > 4000000.00 5028099.31 0.00
> >
> > The difference is about 8 km. But I cannot see it in QGIS.
> >
> > Jakub suggested that it could be the simplification. I disabled it in
> the settings. No difference.
> >
> > Is QGIS doing something strange? Can I disable it?
> >
> > If I select EPSG:4326 instead of EPSG:3857, then you can see clearly the
> two linestrings.
> >
> > Thanks,
> > Javier.
> >
> >
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