You can also build your VRT using QGIS' processing toolbox. On Oct 5, 2017 7:26 PM, "Régis Haubourg" <regis.haubo...@gmail.com> wrote:
> GDAL is probably installed on your system, you need it for QGIS :) > > something like this should do the trick in a linux bash or from the > osgeo4w embedded shells (either windows or msys ) > gdalbuildvrt all_ecw.vrt *.tiff -a_srs "EPSG:yourSridHere" > See http://geoserver.geo-solutions.it/edu/en/raster_ > data/advanced_gdal/example1.html for instance > > > > > 2017-10-05 14:19 GMT+02:00 Patrick Dunford <blackwhite...@gmail.com>: > >> I probably don't have the technical means to use that. >> >> On 06/10/17 01:12, Régis Haubourg wrote: >> >> here it is: http://www.gdal.org/gdal_vrttut.html >> >> you can assemble raster of vector files, apply filters, reprojections, >> build pyramids to avoid fetching the lowest resolution data. This is pretty >> efficient. >> >> Cheers >> Régis >> >> 2017-10-05 14:10 GMT+02:00 Patrick Dunford <blackwhite...@gmail.com>: >> >>> I don't know what this is. >>> >>> On 06/10/17 01:08, Régis Haubourg wrote: >>> >>> Hi Patrick , >>> did you consider using GDAL VRT to avoid opening so much files? >>> In my experience, this works well. >>> Régis >>> >>> >>> 2017-10-05 13:48 GMT+02:00 Patrick Dunford <blackwhite...@gmail.com>: >>> >>>> Some time ago in a discussion of a particular bug a contributor >>>> expressed concern that the refresh of background rasters (aerial >>>> photography) in Windows was too slow. >>>> >>>> Maybe this is the reason that recent versions of master appear to be >>>> loading all of the background imagery into memory (I use a master from >>>> January this year to work around issues with later ones, and that master >>>> does not have this feature). >>>> >>>> Unfortunately if there are a lot of rasters then the memory demand is >>>> excessive and unsurprisingly slows down the computer negating any purported >>>> benefit of caching. >>>> >>>> As an example a project I am currently working on has about 900 aerial >>>> photo images (GeoJpeg). When the layer is turned on for display, Qgis >>>> requires about 46 GB of virtual memory. Since my computer only has 24 GB of >>>> physical memory, it is required to dip into the swap space considerably. >>>> Even with 60 GB of swap space on an SSD, the swapping needed to refresh the >>>> canvas is substantial and dramatically reduces performance resulting in >>>> substantial delays. Compare with the January master referred to above which >>>> only requires about 7 GB of virtual memory total with the aerial photo >>>> layer displayed. The time needed to refresh the canvas is less than 1 >>>> second, most of the time. >>>> >>>> I know that the canvas refresh in Windows with aerial photos can be >>>> substantially slower than in Linux. This does not affect me, because I >>>> don't use Windows now that I have a stable platform for running an older >>>> Linux master alongside the most recent one. What I do know is that the >>>> memory demands are making it difficult to evaluate the recent masters. I >>>> need some kind of setting to turn this caching off. With the aerial photo >>>> layer turned off, the memory usage of the current master is about the same >>>> as the old one, and it's much quicker to update. >>>> >>>> >>>> _______________________________________________ >>>> QGIS-Developer mailing list >>>> QGIS-Developer@lists.osgeo.org >>>> List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer >>>> Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer >>> >>> >>> >>> >> >> > > _______________________________________________ > QGIS-Developer mailing list > QGIS-Developer@lists.osgeo.org > List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer > Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer >
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