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

[Even Rouault]

On mercredi 29 août 2018 21:50:25 CEST Andreas Neumann wrote:
> Hi Even,
> 
> Thank you very much for the detailed analysis of the spatial filter
> issue we have here, that causes the disappearing grid lines.
> 
> Would you be interested to work on fixing this issue on our bug fixing
> program? This problem annoyed me for quite some time. And it appears on
> different world projections.

I might have a look at this, if nobody else more familiar with the relevant 
code looks at it. Is there a ticket filed about that ?

Even

> 
> Thanks,
> 
> Andreas
> 
> Am 28.08.2018 um 22:19 schrieb Even Rouault:
> > 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
> > ogrlayer.cpp:1223
> > 
> > 
> > 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)"
> > 
> > Even
> > 
> >> 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
> >>> 
> >>> 
> >>> _______________________________________________
> >>> QGIS-Developer mailing list
> >>> QGIS-Developer at lists.osgeo.org
> >>> List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer
> >>> Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer


-- 
Spatialys - Geospatial professional services
http://www.spatialys.com