Thank you Stefan for your help! I figured what happens. v.strds.stats with default method produces in my case a corrupted output, topology is there but there's no table associated to it. If I specify method=average, I do obtain the table, and the mystery is solved: some of the polygons fall into a nodata (due to cloud mask). If anyone else can reproduce the corrupted table issue, I can file a ticket for that.
Thanks for help! Regards, On Thu, Feb 7, 2019 at 12:33 PM Stefan Blumentrath < stefan.blumentr...@nina.no> wrote: > Hi Madi, > > > > With this combination (polygon size vs. raster resolution), the shape of > the polygons can be an issue (narrow areas that do not cover the center of > any pixel). > > > > Debugging should be simple with v.db.select or v.extract. > > > > Areas that did not get rasterized should be NULL in the column with > statistics computed with v.rast.stats. > > > > In verbose mode v.rast.stats (or probably even v.to.rast) should probably > give a more informative Warning message (e.g. listing categories not > rasterized). It also would help if you can rasterize the areas yourself and > provide a raster with categories as (optional) input to v.rast.stats… > > > > For high resolution data like yours, the speed improvement of multiple > raster input might help quite a bit esp. with many maps in the time series. > Will see if I can come up with a patch rather soon… > > > > Cheers > > Stefan > > > > *From:* Margherita Di Leo <direg...@gmail.com> > *Sent:* torsdag 7. februar 2019 10:37 > *To:* Stefan Blumentrath <stefan.blumentr...@nina.no> > *Cc:* Veronica Andreo <veroand...@gmail.com>; grass-user < > grass-user@lists.osgeo.org> > *Subject:* Re: [GRASS-user] sample a strds at specific locations (areas) > > > > Hi, > > > > thank you for your replies. To give a little more context: I selected my > polygon areas to be > 0.5 ha each (this would be 50000 mq if I'm not > mistaken) and I'm sampling NDVI maps at 10m resolution (the region being > the same as NDVI maps). So I think I need an idea on how to debug the areas > that were excluded to check them individually to see what could be the > problem... > > Regarding the alignment problem, if I understand it correctly: if the > polygon doesn't include the *center* of the raster beneath it, can't > retrieve the value and the polygon is discarded? But a value exists, so it > would be correct that it returned a value in any case. But I admit I don't > have a full grasp of the problem. > > > > On Wed, Feb 6, 2019 at 10:52 PM Stefan Blumentrath < > stefan.blumentr...@nina.no> wrote: > > Hi Vero, > > > > I think there is a little misunderstanding. > > v.rast.stats did not change it behaviour with regards to the computational > region (at least not if only one raster map is used). The alignment to the > input raster (resolution) has been around since the module got ported to > Python (like 10 years ago): > > > https://trac.osgeo.org/grass/browser/grass/trunk/scripts/v.rast.stats/v.rast.stats.py?rev=33522#L148 > > > > So, adding a flag for skipping the alignment was more an idea for an > enhancement that allows the behaviour you seem to prefer (too). > > > > Cheers > > Stefan > > > > *From:* Veronica Andreo <veroand...@gmail.com> > *Sent:* onsdag 6. februar 2019 21:38 > *To:* Stefan Blumentrath <stefan.blumentr...@nina.no> > *Cc:* Margherita Di Leo <direg...@gmail.com>; grass-user < > grass-user@lists.osgeo.org> > *Subject:* Re: [GRASS-user] sample a strds at specific locations (areas) > > > > I had a similar problem some time ago, just that it was not raster > resolution, but region resolution that I changed to solve my problem (see > this thread and MM's answer: > http://osgeo-org.1560.x6.nabble.com/v-to-rast-for-polygons-not-overlapping-center-of-raster-cell-td5355686.html#a5355729 > ) > > > > IIUC, MM's proposed solution to my case then does not work anymore because > v.to.rast call inside v.rast.stats is affected by the region alignment to > the raster to be queried. So, the solution is indeed now, to change raster > resolution... ? Then the region would be aligned to it (them)? > > > > If one has large areas or long time series and has to resample all rasters > to get smallish polygons rasterized, I do not see the advantage of this new > behavior... but maybe I'm missing something > > > > Cheers, > > Vero > > > > El mié., 6 feb. 2019 16:54, Stefan Blumentrath <stefan.blumentr...@nina.no> > escribió: > > Ciao Madi, Vero, > > > > Starting with GRASS 7.6, also centroids are used to get the raster > representation of your area vector map. That increases the likelihood of > smaller areas to be rasterized. > > Increasing the resolution of the current region alone does not help, > because v.rast.stats temporarily changes the computational region to align > with the input raster map(s) (see also: > https://trac.osgeo.org/grass/ticket/3523 and > https://trac.osgeo.org/grass/ticket/3598 for discussion) Even if the > first ticket is closed, comments are welcome. > > It might make sense to add a flag to v.rast.stats like in r.slope.aspect > to not align the computational region. > > > > Furthermore, with regards to efficiency, v.strds.stats could gain some > speed if multi-raster support in v.rast.stats - added in G 7.6 - would be > handed down to the addon. Might almost double the speed for larger STRDS… > > > > Cheers > > Stefan > > > > *From:* grass-user <grass-user-boun...@lists.osgeo.org> *On Behalf Of > *Veronica > Andreo > *Sent:* onsdag 6. februar 2019 17:20 > *To:* Margherita Di Leo <direg...@gmail.com> > *Cc:* GRASS user list <grass-user@lists.osgeo.org> > *Subject:* Re: [GRASS-user] sample a strds at specific locations (areas) > > > > Hi Madi > > > > El mié., 6 feb. 2019 a las 16:31, Margherita Di Leo (<direg...@gmail.com>) > escribió: > > I have a question regarding v.strds.stats. I get the following warning > message: > > > > WARNING: Not all vector categories converted to raster. Converted 120 of > 265. > > > > What could be the reason for that? > > > > Some vector areas might not be converted because they are too small with > respect to the pixel size that you try to query. Others will tell better > but I think the polygon must overlap the center of the pixel in order to be > converted into raster. One solution could be to resample your rasters to a > higher resolution. > > HTH, > > Vero > > > > -- > > Margherita Di Leo > -- Margherita Di Leo
_______________________________________________ grass-user mailing list grass-user@lists.osgeo.org https://lists.osgeo.org/mailman/listinfo/grass-user
