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

Mon Mar 5 05:48:16 PST 2018

Ok, Thanks for your detailed explanation!

On 5 Mar 2018 15:41, "Kristian Evers" <kreve at sdfe.dk> wrote:

It would be nice if the Web Mercator was never introduced to the EPSG
register,
but nothing we can do about that now. The problem has indeed seeped into
PROJ by mistake (will be fixed with 5.0.1). Unfortunately the internal
architecture
in PROJ makes it quite hard to handle both the Web Mercator and treat other
(not so common) transformations with a shift in ellipsoid correctly. Luckily
this is only a problem in the old API. The new API introduced in 5.0.0 is
more
flexible and can handle both correctly. It will be a while before that is
easily
available for downstream projects such as QGIS.

/Kristian

-----Oprindelig meddelelse-----
Fra: Qgis-user [mailto:qgis-user-bounces at lists.osgeo.org] På vegne af
Benjamin Ducke
Sendt: 5. marts 2018 11:58
Til: qgis-user <qgis-user at lists.osgeo.org>
Emne: Re: [Qgis-user] Pseudo Mercator(OSM) @ EPSG:4326 bug?

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>> 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>
>
>     /Kristian
>
>     -----Oprindelig meddelelse-----
>     Fra: Qgis-user [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>
>     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>
>
>     ?
>
>     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/%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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