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<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US">Some experience with GDAL VRT and GRASS:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US">Be aware of:
<a href="https://trac.osgeo.org/grass/ticket/2837">https://trac.osgeo.org/grass/ticket/2837</a><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US">Also note also that esp. for integer rasters, compression of one big raster map can be much better than with lots of smaller tiles [1]. So I guess cases where patching by means of a
virtual raster is most beneficial is in intermediate steps of an analysis chain…<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US">Cheers<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US">Stefan<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US">1: <a href="https://www.mail-archive.com/grass-user@lists.osgeo.org/msg33125.html">
https://www.mail-archive.com/grass-user@lists.osgeo.org/msg33125.html</a><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> grass-user <grass-user-bounces@lists.osgeo.org>
<b>On Behalf Of </b>Markus Metz<br>
<b>Sent:</b> fredag 1. juni 2018 08:24<br>
<b>To:</b> Markus Neteler <neteler@osgeo.org><br>
<b>Cc:</b> GRASS user list <grass-user@lists.osgeo.org><br>
<b>Subject:</b> Re: [GRASS-user] GRASS GIS virtual raster data sets<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-GB"><br>
<br>
On Thu, May 31, 2018 at 10:41 PM, Markus Neteler <</span><a href="mailto:neteler@osgeo.org"><span lang="EN-GB">neteler@osgeo.org</span></a><span lang="EN-GB">> wrote:<br>
><br>
> On Wed, May 30, 2018 at 4:21 PM, Nikos Alexandris<br>
> <</span><a href="mailto:nik@nikosalexandris.net"><span lang="EN-GB">nik@nikosalexandris.net</span></a><span lang="EN-GB">> wrote:<br>
> > Dears,<br>
> ><br>
> > I guess there is no GRASS GIS built-in implementation of GDAL's VRT<br>
> > concept (several related ideas listed<br>
> > </span><a href="https://trac.osgeo.org/grass/wiki/Grass8Planning"><span lang="EN-GB">https://trac.osgeo.org/grass/wiki/Grass8Planning</span></a><span lang="EN-GB">).<br>
><br>
> I wish to see that as well (and discussed it a while ago with our dear<br>
> Markus M...<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-GB">I have an idea about how to implement native GRASS VRT, but no resources to do so...<o:p></o:p></span></p>
</div>
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<p class="MsoNormal"><span lang="EN-GB">><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-GB">> GRASS GIS would gain a lot having the possibility to virtually mosaik<br>
> raster tiles.<br>
><br>
> > Once past some compilation issues, I am about to try to build a VRT<br>
> > based on native GRASS GIS raster maps (using GDAL with support for GRASS<br>
> > GIS raster data), then link to this VRT via `r.external` from inside a<br>
> > regular Mapset.<br>
><br>
> Interesting, will that not cause loop dependencies?<br>
><br>
> I thought of true internal support for that to avoid loops.<br>
><br>
> > As a side-note, an image without pre-computed statistics stored in its<br>
> > metadata, will require some time for `r.external` to work. The latter<br>
> > likely forces this computation.<br>
> ><br>
> > Perhaps it is faster to use `gdalinfo`'s '-stats' option to do this.<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-GB">It will not be faster because you would need gdalinfo -mm -stats in order to get the actual values, not an approximation. Sometimes an approximation of the stats based on a subsample is stored in the metadata of a raster.<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-GB">> ><o:p></o:p></span></p>
</div>
<p class="MsoNormal"><span lang="EN-GB">> > I wonder if there are smart ways to build, in GRASS GIS' data base,<br>
> > a larger virtual raster map out of many smaller ones, without the need<br>
> > to actually patch the latter ones in a new raster map file.<br>
><br>
> I guess that some C code needs to be touched in lib/raster/ to allow<br>
> for a VRT-style raster concept.<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-GB">Yes.<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-GB">> Maybe we missed the current GSoC train for this idea :)><o:p></o:p></span></p>
<div>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-GB">The tricky part is to get a qualified student for this project.<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
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<p class="MsoNormal">Markus M<o:p></o:p></p>
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<div>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">> markusN<o:p></o:p></p>
</div>
<p class="MsoNormal" style="margin-bottom:12.0pt">> _______________________________________________<br>
> grass-user mailing list<br>
> <a href="mailto:grass-user@lists.osgeo.org">grass-user@lists.osgeo.org</a><br>
> <a href="https://lists.osgeo.org/mailman/listinfo/grass-user">https://lists.osgeo.org/mailman/listinfo/grass-user</a><o:p></o:p></p>
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