[Qgis-user] How QGIS handles on-the-fly projection?

[Ujaval Gandhi]

There is specific code for QGIS to handle transformation between EPSG:3857 and
EPSG:4326 due to EPSG:3857 not being valid at poles. See the comments at
https://github.com/qgis/QGIS/blob/master/src/core/proj/
qgscoordinatetransform.cpp#L585
[https://mailtrack.io/l/3b9f0f5d1696eca585d974232660d3c67b88aeef?url=https%3A%2F%2Fgithub.com%2Fqgis%2FQGIS%2Fblob%2Fmaster%2Fsrc%2Fcore%2Fproj%2Fqgscoordinatetransform.cpp%23L585&u=8747767&signature=160430bc5e3e39b2]


You should not be using EPSG:3857 for datasets that span more than +/-85 degrees
latitude. If you want to display global datasets in a projected CRS, try Equal
Earth (EPSG:8857). 


---
Ujaval Gandhi
Spatial Thoughts
www.spatialthoughts.com
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[data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=3D]
On Wed, Feb 28, 2024 at 6:32 PM Régis Haubourg via QGIS-User
<qgis-user at lists.osgeo.org> wrote:


> Hi ,
> 
> 
> the render in QGIS is always done on the fly, there is no caching of
> prerendered data, except for online webservices. 
> 
> PROJ is indeed the library doing the job. 
> 
> Régis
> 
> 
> On 27/02/2024 14:00, Ionut Ungurianu via QGIS-User wrote:
> 
> 
> > Hi!
> > 
> > I’m looking to understand  in a more advanced way how QGIS handles on-the-fly projection.
> > In my case I set the project crs in epsg:3857 and I have a raster layer in epsg:4326. In this case what is happening with the layer?
> > 
> > I know that it’s based on PROJ, but I’m wondering what is actually happening in the back-end, what is happening in the map canvas, how is the layer render, is being resampled every time you pan the map or do zoom in/out ...
> > 
> > So, if someone can help with a detailed explanation, I would really appreciate it.
> > 
> > Thanks in advance!
> > 
> > All the best!
> > Ionut
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