[QGIS-Developer] proj5 and epsg:3857 ?

[Even Rouault]

Kristian 

your are certainly not too blame at à personal level. this is the kind of this that happen inevitably given a certain amount of changes . i'm surprised that the GDAL autotest suite did not catch this but the cause is likely that the GDAL c++ class that wraps proj has an optimisation when transforming from 3857 to 4326. if you transform an array of points of the same northing they have the same latitude. this is a common case for gdalwarp internal mechanics. thus you need to do trigonometric computations just once! so proj is completely short circuited for that case. 

i'll add a test for the reverse transformation that uses proj (could receive a similar optimisation but isnt needed when warping unrectified imagery with RPC to WebMercator)

Even 
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<div>-------- Message d'origine --------</div><div>De : Kristian Evers <kreve at sdfe.dk> </div><div>Date :09/03/2018  10:39  (GMT+01:00) </div><div>À : qgis-developer at lists.osgeo.org </div><div>Cc :  </div><div>Objet : Re: [QGIS-Developer] proj5 and epsg:3857 ? </div><div>
</div>All,

Just to follow up on this, since I am the one to blame for this error to show up.
Both by introducing it in the first place and by releasing 5.0.0 with this as a 
known bug. The bug was introduced to fix a range of other potential issues.
Unfortunately the architecture in the old API of PROJ makes it almost impossible
to both treat other transformations correct while also treating the Web Mercator
correctly. The problem is that the Web Mercator is, geodetically speaking, an
abomination of a transformation. It basically defies all logic. This has been known
for a long time of course. Unfortunately no tests were set up to catch this specific
transformation. I'll make sure to at least add Jeff's test case.

Regarding releasing the library with this bug included. The cause of the problem
was determined very close to the release time (which was announced a month
ahead). We did go through six release candidates. The vote for promotion of RC6
to final release was already started so I decided to release anyway since I did not
want to prolong the process even more. I am sure this bug would have been
discovered earlier if all of you QGIS developers would not have been super
busy with QGIS 3.0 and had had some spare time to test the PROJ release
candidates. I am not blaming anyone, it is just a case of bad timing. Not much
we can do about that really. Had I been in your shoes I would not have prioritized
testing other software either, that's for sure!

The problem has an easy fix. Remove the "+nadgrids=@null" from your epsg
init-file and the transformation should work as expected. A proper fix will
come in 5.0.1 around April 1st.  I hope you all will continue to use PROJ 5.0.0
in your development builds despite this problem so other bugs can see the
light of day and be fixed in time for 5.0.1.

I completely agree with not requiring 5.0.0 as a dependency for QGIS. Hopefully
5.0.1 is a little better so that can be the recommended PROJ version for QGIS.

I would like to encourage you to take a look at the release schedule [0] for the next
couple of years. We are going to change things in order to keep up with current
geodetic developments in countries such as Australia, USA and Iceland.  It will
affect QGIS sooner or later. I am of course happy to provide assistance where
needed.

/Kristian

[0] https://github.com/OSGeo/proj.4/milestones?direction=asc&sort=due_date&state=open

-----Oprindelig meddelelse-----
Fra: QGIS-Developer [mailto:qgis-developer-bounces at lists.osgeo.org] På vegne af Jeff McKenna
Sendt: 8. marts 2018 12:50
Til: qgis-developer at lists.osgeo.org
Emne: Re: [QGIS-Developer] proj5 and epsg:3857 ?

Hi Richard,

You can read my report to the PROJ team while I was testing the release 
candidates: 
http://lists.maptools.org/pipermail/proj/2018-February/008082.html 
Sorry, it is a long thread.  I was trying to prevent (this here in QGIS, 
for example) from happening, before the release.

-jeff





On 2018-03-07 4:13 PM, Richard Duivenvoorde wrote:
> Hi,
> 
> Having a geocoder plugins, in 2.18 (still proj4), I can geocode my home
> address, and reproject the epsg:28992 coordinate fine to epsg:3857.
> 
> But since last week Debian has proj5, and the same plugin (in QGIS
> master) now is off by 500 meter.
> 
> To reproduce: if you have a QGIS 2.18 with proj4, and the
> 'pdokservicesplugin' geocode the zip+number: '2022zj 23' in for example
> the osm xyz layer.
> Now do the same in QGIS master with proj5 and you'll see it is not going
> to the right/same position.
> 
> Another observation: IN 2.18 I can 'reproject' the OSM layer to for
> example epsg:4326, and still see (little distorted) map.
> But in QGIS3/proj5 I do not see a map anymore.
> 
> Anybody else seeing this?
> 
> Regards,
> 
> Richard Duivenvoorde
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