[Qgis-user] Pseudo Mercator(OSM) @ EPSG:4326 bug?

[Kristian Evers]

If you are using PROJ 5.0.0 it is likely to be caused by this known issue:

https://github.com/OSGeo/proj.4/issues/834

/Kristian

-----Oprindelig meddelelse-----
Fra: Qgis-user [mailto:qgis-user-bounces at lists.osgeo.org] På vegne af Benjamin Ducke
Sendt: 5. marts 2018 10:18
Til: qgis-user at lists.osgeo.org
Emne: Re: [Qgis-user] Pseudo Mercator(OSM) @ EPSG:4326 bug?

Could it be this:

http://proj4.org/faq.html#changing-ellipsoid-why-can-t-i-convert-from-wgs84-to-google-earth-virtual-globe-mercator

?

The further you get from the equator, the more
obvious the discrepancy on the Y axis.

Best,

Ben

On 05/03/18 09:38, Idan Miara wrote:
> Hi,
> 
> I might have found a bug related to transformation of layer CRS
> from EPSG:3857 (Pseudo Mercator) to EPSG:4325 (WGS84 Geo).
> I'm attaching a few images to demonstrate the issue. 
> 
> Steps to reproduce:
> I've load OSM in QGIS 2.18.16 (or 3.0,same results)
> via Tile Server (XYZ):
> https://a.tile.openstreetmap.org/{z}/{x}/{y}.png
> <https://a.tile.openstreetmap.org/%7Bz%7D/%7Bx%7D/%7By%7D.png>
> 
> Then set the CRS to EPSG:4326 (WGS84 Geo)
> 
> I've also loaded a global bounds vector layer (EPSG:4326)
> 
> In scale 1:100,000,000 it seems that the Pseudo Mercator transformation
> to 4326 is flawed, as the OSM and the vectors are not aligned.
> In scale 1:50,000,000 it seems OK when the extent is the middle of the
> map, but if I pan to the south/north it seems distorted again.
> 
> If I set the project CRS to EPSG:3857 (Pseudo Mercator) as the CRS of
> the OSM tiles, so the vector layer would be transformed instead, then
> they do align in all scales (picture with the blue vector layer).
> 
> It doesn't look like a problem of precision of the transformation from
> 3857 to 4325 because it's way off. 
> Is that a bug or am I missing something here?
> 
> Kind regards,
> Idan.
> 
> 
> 
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-- 
Dr. Benjamin Ducke
Deutsches Archäologisches Institut (DAI)
Zentrale Berlin, IT-Referat
* Projekt "Stunde Null" *
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