[GRASS-user] GRASS GIS virtual raster data sets

[Markus Metz]

On Thu, May 31, 2018 at 10:41 PM, Markus Neteler <neteler at osgeo.org> wrote:
>
> On Wed, May 30, 2018 at 4:21 PM, Nikos Alexandris
> <nik at nikosalexandris.net> wrote:
> > Dears,
> >
> > I guess there is no GRASS GIS built-in implementation of GDAL's VRT
> > concept (several related ideas listed
> > https://trac.osgeo.org/grass/wiki/Grass8Planning).
>
> I wish to see that as well (and discussed it a while ago with our dear
> Markus M...

I have an idea about how to implement native GRASS VRT, but no resources to
do so...
>
> GRASS GIS would gain a lot having the possibility to virtually mosaik
> raster tiles.
>
> > Once past some compilation issues, I am about to try to build a VRT
> > based on native GRASS GIS raster maps (using GDAL with support for GRASS
> > GIS raster data), then link to this VRT via `r.external` from inside a
> > regular Mapset.
>
> Interesting, will that not cause loop dependencies?
>
> I thought of true internal support for that to avoid loops.
>
> > As a side-note, an image without pre-computed statistics stored in its
> > metadata, will require some time for `r.external` to work. The latter
> > likely forces  this computation.
> >
> > Perhaps it is faster to use `gdalinfo`'s '-stats' option to do this.

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.

> >
> > I wonder if there are smart ways to build, in GRASS GIS' data base,
> > a larger virtual raster map out of many smaller ones, without the need
> > to actually patch the latter ones in a new raster map file.
>
> I guess that some C code needs to be touched in lib/raster/ to allow
> for a VRT-style raster concept.

Yes.

> Maybe we missed the current GSoC train for this idea :)>

The tricky part is to get a qualified student for this project.

Markus M

> markusN
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