[GRASS-user] GRASS GIS virtual raster data sets

[Stefan Blumentrath]

Hi,

Some experience with GDAL VRT and GRASS:

Be aware of: https://trac.osgeo.org/grass/ticket/2837

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…

Cheers
Stefan

1: https://www.mail-archive.com/grass-user@lists.osgeo.org/msg33125.html


From: grass-user <grass-user-bounces at lists.osgeo.org> On Behalf Of Markus Metz
Sent: fredag 1. juni 2018 08:24
To: Markus Neteler <neteler at osgeo.org>
Cc: GRASS user list <grass-user at lists.osgeo.org>
Subject: Re: [GRASS-user] GRASS GIS virtual raster data sets



On Thu, May 31, 2018 at 10:41 PM, Markus Neteler <neteler at osgeo.org<mailto:neteler at osgeo.org>> wrote:
>
> On Wed, May 30, 2018 at 4:21 PM, Nikos Alexandris
> <nik at nikosalexandris.net<mailto: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
> _______________________________________________
> grass-user mailing list
> grass-user at lists.osgeo.org<mailto:grass-user at lists.osgeo.org>
> https://lists.osgeo.org/mailman/listinfo/grass-user
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/grass-user/attachments/20180601/9623afc2/attachment-0001.html>