[GRASS-user] sample a strds at specific locations (areas)
Margherita Di Leo
diregola at gmail.com
Wed Aug 14 07:58:47 PDT 2019
Ciao Vero,
indeed that was my first guess about the cause, I double checked but the
region is properly set.
Sorry for late answer, I'm travelling these days with bad to none internet
connection.
Thanks
On Mon, Aug 12, 2019 at 4:23 PM Veronica Andreo <veroandreo at gmail.com>
wrote:
> Ciao Madi,
>
> is the region properly set?
>
> @Stefan in this case, if the centroid only is rasterized, then stats will
> also belong to that pixel only? AFAIU, we don't want that, but stats for
> the area. Or rasterization of centroid would have a different effect?
>
> best,
> Vero
>
> El vie., 9 ago. 2019 a las 21:29, Stefan Blumentrath (<
> Stefan.Blumentrath at nina.no>) escribió:
>
>> Ciao Margherita,
>>
>>
>>
>> This is strange. The area (I assume it is an area though your image just
>> shows the boundary) should definitely get rasterized and thus analysed in
>> v.strds.stats.
>>
>>
>>
>> Can you post the output of:
>>
>> v.category input=big_areas2 at PERMANENT option=report
>>
>> and
>>
>> v.info test -t ?
>>
>>
>>
>> I double-checked, and the possibility to rasterize also centroids is
>> unfortunately not handed down to v.rast.stats (and moduls build ontop of
>> that, like v.strds.stats). This should be done for GRASS version >= 7.6
>> (e.g. by adding centroids to the default selection of types to rasterize in
>> v.to.rast) and is probably worth a ticket.
>>
>>
>>
>> Cheers
>>
>> Stefan
>>
>>
>>
>> *From:* Margherita Di Leo <diregola at gmail.com>
>> *Sent:* fredag 9. august 2019 16:51
>> *To:* Veronica Andreo <veroandreo at gmail.com>
>> *Cc:* Stefan Blumentrath <Stefan.Blumentrath at nina.no>; grass-user <
>> grass-user at lists.osgeo.org>
>> *Subject:* Re: [GRASS-user] sample a strds at specific locations (areas)
>>
>>
>>
>> Hi,
>>
>>
>>
>> excuse me if I return on this. I have again the same problem, of v.strds.stats
>> skipping lots of polygons. Now I'm using a brand new dataset - strds of
>> Sentinel 1 and a brand now vector of polygons - and the skipped polygons
>> are not narrow, I'm sure that there are cells centroids in it. See for
>> example screenshot attached, depicting a polygon that was skipped. I also
>> tried to run v.strds.stats on that polygon alone, like:
>>
>>
>>
>> v.strds.stats in=big_areas2 at PERMANENT where="cat == '1'"
>> strds=db_cross_pol at scaled out=test method=average
>>
>>
>>
>> v.info test
>>
>> +----------------------------------------------------------------------------+
>> | Name: test
>> |
>> | Mapset: stats
>> |
>> | Location: S1
>> |
>> | Database: /media/madi/TOSHIBA EXT/S1/grassdata
>> |
>> | Title: Output from v.patch
>> |
>> | Map scale: 1:1
>> |
>> | Name of creator: madi
>> |
>> | Organization:
>> |
>> | Source date: Fri Aug 9 12:12:49 2019
>> |
>> | Timestamp (first layer): none
>> |
>>
>> |----------------------------------------------------------------------------|
>> | Map format: native
>> |
>>
>> |----------------------------------------------------------------------------|
>> | Type of map: vector (level: 2)
>> |
>> |
>> |
>> | Number of points: 0 Number of centroids: 0
>> |
>> | Number of lines: 0 Number of boundaries: 0
>> |
>> | Number of areas: 0 Number of islands: 0
>> |
>> |
>> |
>> | Map is 3D: No
>> |
>> | Number of dblinks: 0
>> |
>> |
>> |
>> | Projection: UTM (zone 34)
>> |
>> |
>> |
>> | N: 0 S: 0
>> |
>> | E: 0 W: 0
>> |
>> |
>> |
>> | Digitization threshold: 0
>> |
>> | Comment:
>> |
>> |
>> |
>>
>> +----------------------------------------------------------------------------+
>>
>> I confess I hadn't look much further into it because I thought it was a
>> problem with nodata, but this is not the case and I think it's worth of
>> further investigation. Thanks for any pointers.
>>
>>
>>
>> Regards,
>>
>>
>>
>>
>>
>> On Thu, Feb 7, 2019 at 4:10 PM Margherita Di Leo <diregola at gmail.com>
>> wrote:
>>
>> Thanks for testing, Vero. I assume it's due to a local problem then.
>>
>>
>>
>> Cheers,
>>
>>
>>
>> On Thu, Feb 7, 2019 at 3:29 PM Veronica Andreo <veroandreo at gmail.com>
>> wrote:
>>
>> 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
>>
>>
>>
>> --
>>
>> Margherita Di Leo
>>
>>
>>
>>
>> --
>>
>> Margherita Di Leo
>>
>
--
Margherita Di Leo
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/grass-user/attachments/20190814/6f69a70c/attachment-0001.html>
More information about the grass-user
mailing list