[Qgis-user] Pseudo Mercator(OSM) @ EPSG:4326 bug?
Benjamin Ducke
benducke at fastmail.fm
Mon Mar 5 03:14:39 PST 2018
On 05/03/18 12:12, Idan Miara wrote:
> Forgive me for not understanding the fine details of the flaws in Web
> Mercator.
> Fast forward back the reality where many tile servers provide tiles in
> 3857, and 4326 to 3857 seems OK, how can I debug this issue in the
> inverse transformation?
I don't think you can. The "fix" would be to force QGIS
to treat all data as WGS:4326 (see again the link I
provided in my first answer).
That seems to be the reality of this mess.
Best,
Ben
>
> I guess I could export the OSM map in 3857 to TIF to rule out the
> possibility that the problem in the tile server, so that I can be sure
> that QGIS is transforming the raster?
>
>
> On 5 Mar 2018 12:57, "Benjamin Ducke" <benducke at fastmail.fm
> <mailto:benducke at fastmail.fm>> wrote:
>
> It all boils down to the same fundamental issue:
> Google Earth/Maps, etc. _pretend_ that their data
> is WGS84, but they do not implement the WGS84
> ellipsoid. So when tools like PROJ attempt to
> do the _technically correct_ thing and perform
> a datum transformation WGS84 <-> Web Mercator, then
> data ends up in the wrong place. If they don't,
> then people get confused, because the two systems
> are different. This confusion is now seeping into
> PROJ and from there into higher-level apps like
> QGIS. I wish EPSG had never included Web Mercator into
> its database. Then we could just ignore all of this,
> use EPSG:4326 and live in blissful ignorance.
>
> On 05/03/18 11:42, Idan Miara wrote:
> > Thanks for you reply,
> > Since the problem exist also in 2.18 I don't think it is related with
> > PROJ 5.0, Right?
> >
> > On 5 Mar 2018 12:39, "Kristian Evers" <kreve at sdfe.dk
> <mailto:kreve at sdfe.dk>
> > <mailto:kreve at sdfe.dk <mailto:kreve at sdfe.dk>>> wrote:
> >
> > 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
> <https://github.com/OSGeo/proj.4/issues/834>
> > <https://github.com/OSGeo/proj.4/issues/834
> <https://github.com/OSGeo/proj.4/issues/834>>
> >
> > /Kristian
> >
> > -----Oprindelig meddelelse-----
> > Fra: Qgis-user [mailto:qgis-user-bounces at lists.osgeo.org
> <mailto:qgis-user-bounces at lists.osgeo.org>
> > <mailto:qgis-user-bounces at lists.osgeo.org
> <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
> <mailto:qgis-user at lists.osgeo.org> <mailto:qgis-user at lists.osgeo.org
> <mailto: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
> <http://proj4.org/faq.html#changing-ellipsoid-why-can-t-i-convert-from-wgs84-to-google-earth-virtual-globe-mercator>
> >
> <http://proj4.org/faq.html#changing-ellipsoid-why-can-t-i-convert-from-wgs84-to-google-earth-virtual-globe-mercator
> <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/{z}/{x}/{y}.png>
> > <https://a.tile.openstreetmap.org/{z}/{x}/{y}.png
> <https://a.tile.openstreetmap.org/{z}/{x}/{y}.png>>
> > >
> <https://a.tile.openstreetmap.org/%7Bz%7D/%7Bx%7D/%7By%7D.png
> <https://a.tile.openstreetmap.org/%7Bz%7D/%7Bx%7D/%7By%7D.png>
> > <https://a.tile.openstreetmap.org/%7Bz%7D/%7Bx%7D/%7By%7D.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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> --
> Dr. Benjamin Ducke
> Deutsches Archäologisches Institut (DAI)
> Zentrale Berlin, IT-Referat
> * Projekt "Stunde Null" *
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--
Dr. Benjamin Ducke
Deutsches Archäologisches Institut (DAI)
Zentrale Berlin, IT-Referat
* Projekt "Stunde Null" *
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