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 <phid...@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@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. >> >> _______________________________________________ >> QGIS-User mailing list >> QGIS-User@lists.osgeo.org >> List info: https://lists.osgeo.org/mailman/listinfo/qgis-user >> Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user >> > > > -- > Srdačan pozdrav / Kind regards, > Vedran Stojnović. >
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