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

[Richard Duivenvoorde]

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.
> 
> 
> _______________________________________________
> QGIS-User mailing list
> QGIS-User at lists.osgeo.org
> List info: https://lists.osgeo.org/mailman/listinfo/qgis-user
> Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user