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

[Vedran Stojnović]

Hi Javier,

if I understood properly what you are trying to achieve - you want overlap
drawings (coordinates) from two coordinate systems as one image without
transformation - then you need to override "Assigned coordinate system"
(Right click on Layer -> Properties -> Source, and change Assigned
Coordinate Reference System.
That way QGIS will think that these two datasets are in the same coordinate
system and won't transform them "On the fly", otherwise it will reproject
all data to a coordinate system currently set in project properties - see
attached image.

uto, 22. srp 2025. u 17:03 Javier Jimenez Shaw via QGIS-User <
qgis-user at lists.osgeo.org> napisao je:

> 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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-- 
Srdačan pozdrav / Kind regards,
Vedran Stojnović.
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