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

[Javier Jimenez Shaw]

Hi Vedran

No, I do not want to change the SRS of the geometry. The geometries are
defined in EPSG:3395. I want to do the reprojection on the fly with QGIS,
and it is not doing what I would expect.
If I do the reprojection with "ogr2ogr -t_srs EPSG:3857" for both files and
open them in QGIS, then yes, I see the difference. But I thought that QGIS
was able to reproject on the fly.

On Wed, 23 Jul 2025 at 08:40, Vedran Stojnović <phidrho at gmail.com> wrote:

> 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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