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

Thu Feb 7 06:29:07 PST 2019

Hi Stefan and Madi,

Thanks Stefan for the explanations :) Indeed I agree that a flag to avoid
the alignment to input rasters (and just use region settings) sounds good.
I tested what Madi said, but cannot reproduce in the climate NC location
[0]. This is the command I used:

v.strds.stats in=boundary_county where="cat == '261'" strds=tempmean
t_where="start_time >= '2012-01-01'" out=test

to make it faster (it feels indeed kinda slow for the whole vector and full
time series), I selected only one polygon and a range of dates. I get the
table as expected while leaving methods by default.

best,
Vero

[0]
http://courses.ncsu.edu/mea592/common/media/02/nc_climate_spm_2000_2012.zip

El jue., 7 feb. 2019 a las 14:39, Margherita Di Leo (<diregola at gmail.com>)
escribió:

> 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.Blumentrath at 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 <diregola at gmail.com>
>> *Sent:* torsdag 7. februar 2019 10:37
>> *To:* Stefan Blumentrath <Stefan.Blumentrath at nina.no>
>> *Cc:* Veronica Andreo <veroandreo at gmail.com>; grass-user <
>> grass-user at 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.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
>>
>
>
> --
> Margherita Di Leo
>
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