[QGIS-Developer] Disappearing -180 / 180 longitude grid lines on certain zoom levels

Even Rouault even.rouault at spatialys.com
Tue Aug 28 13:19:47 PDT 2018

Yes, seems restricted to reprojection cases

This seems to be an issue with the spatial filter issued to OGR

At the zooms where the lines disappear, there are requests like:

Thread 23 "Thread (pooled)" hit Breakpoint 2, OGR_L_SetSpatialFilterRect 
(hLayer=0x7f81180c90a0, dfMinX=-179.79163612932865, 
dfMinY=-69.446164378986353, dfMaxX=179.90530755284408, 
dfMaxY=78.959077253477474) at ogrlayer.cpp:1223

At the zooms where that work (even when zoomed in), there are like:

Thread 29 "Thread (pooled)" hit Breakpoint 2, OGR_L_SetSpatialFilterRect 
(hLayer=0x7f81180c90a0, dfMinX=-180, dfMinY=-90, dfMaxX=180, dfMaxY=90) at 

I haven't looked at the QGIS code that computes this bounding box, but from my 
experience with gdalwarp which has similar challenges, it is tricky to compute 
a source bounding box from a target bounding box, because sometimes the 
coordinates in the target bounding box do not correspond to a physical point 
on Eath, and hence inverse projection fails. So you have to resort to a grid 
sampling approach, but that makes you miss the exact boundaries. So probably 
that a band-aid fix would be to add some ad-hoc logic, like "if the source SRS 
is long/lat, and the computed extent is almost worldwide, then extend it to 
full worlwide (or do not emit a spatial filter at all)"


> With EPSG:4326 it does not seem to happen, but I tried EPSG:3857 and the
> -180,180 grid lines do appear and disappear at different zoom levels. I
> also confirm this with the Robinson projection with the addition that the
> -90, 90 degree latitude lines also appear and disappear.
> Calvin
> On Tue, Aug 28, 2018 at 3:31 PM, Andreas Neumann <a.neumann at carto.net>
> wrote:
> > Hi again,
> > 
> > For some time already I noticed that, depending on the zoom level, the
> > -180 / 180 degree grid lines appear / disappear in QGIS. This is unrelated
> > to the EqualEarth projection just discussed and also appears on other
> > world
> > projections, like the Robinson projection.
> > 
> > Here is the testfile I used with world (countries) and gridlines:
> > http://www.carto.net/neumann/temp/gridlines.gpkg
> > 
> > Can you reproduce this behaviour?
> > 
> > Any idea why this happens and how this could be fixed?
> > 
> > Thanks,
> > 
> > Andreas
> > 
> > 
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