[GRASS-user] sample a strds at specific locations (areas)

[Margherita Di Leo]

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.Blumentrath at 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 <veroandreo at gmail.com>
> *Sent:* onsdag 6. februar 2019 21:38
> *To:* Stefan Blumentrath <Stefan.Blumentrath at nina.no>
> *Cc:* Margherita Di Leo <diregola at gmail.com>; grass-user <
> grass-user at 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.Blumentrath at 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-bounces at lists.osgeo.org> *On Behalf Of *Veronica
> Andreo
> *Sent:* onsdag 6. februar 2019 17:20
> *To:* Margherita Di Leo <diregola at gmail.com>
> *Cc:* GRASS user list <grass-user at 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 (<diregola at 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
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