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

[Vedran Stojnović]

Hi,

QGIS should do on-the-fly reprojection, but you can count on only the
defined vertices of geometry as far as I know.
You can execute "Densify by interval" or "Densify by count" on source
geometries in source CRS - you can use the "Edit features in place" option.
This will convert your "line" to "linestring" with many smaller segments.
Please check if this solves your problem?

I remember that there was an announcement for some QGIS 3.x version that
this "densification" of vertices is done automatically, but maybe it
stopped working or works for only certain source CRSs (maybe the ones
defined in Lat-Long) - it seems that it works for "Identify tool".

sri, 23. srp 2025. u 11:11 Javier Jimenez Shaw <j1 at jimenezshaw.com> napisao
je:

> 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.
>>
>

-- 
Srdačan pozdrav / Kind regards,
Vedran Stojnović.
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