[GRASS-user] Zonal Raster Statistics with overlapping polygons

[Sitansu Pattnaik]

I had understood that v.rast.stats converted the vector file into raster
and hence the order in which the vectors were drawn, only one value was
generated for the overlapping areas in the raster. This rasterized version
was then used to compute the statistics.
Hence, the overlapping areas matched for polygon 1 and not for polygon 3 as
the overlap area was coded to "1" in the overlap area during raster
conversion.
Regards,
Sitansu

On Mon, Feb 27, 2017 at 5:05 PM, Moritz Lennert <
mlennert at club.worldonline.be> wrote:

> Le Mon, 27 Feb 2017 09:41:11 +0100,
> Leonardo <leonardo at mobygis.com> a écrit :
>
> > Thanks for your answer,
> > I had already thought of a solution of this type,
> > but I'm looking for a cleaner and smarter solution,
> > any hints?!
>
> Well the "smarter" in your case would imply to use non-topological
> vector format, which was decided for GRASS GIS to not be "smarter".
> Yes, this makes some operations a bit more complicated, but you get
> paid back with so much less hassle because of spaghetti data. And
> writing a simple loop over v.rast.stats is not that difficult. The only
> difficulty I see is that v.rast.stats does not have a where= nor cat=
> feature selection parameter. This implies that you have to run
> v.extract each time. Having these parameters would certainly be useful
> and you could file an enhancement wish for those/
>
> I don't know why this would be less "clean"...
>
> The IMHO "cleanest" solution is actually the one that GRASS GIS
> implements: When you import a file with overlapping polygons, GRASS GIS
> creates a vector map in which the overlapping parts of the polygons
> have the category ids of all the polygons that overlap in that area.
> And when you run v.rast.stats on that, it will fill the total of each
> category value in the attribute table.
>
> Attached you can find an image of a test map with polygon 3 overlapping
> both polygon 1 and 2. I also created a second file with only the 2
> non-overlapping polygons. Those two were created in QGIS so as to be
> able to have "nice" topologically incorrect data.
>
> I then imported the two files into GRASS GIS with v.in.ogr and ran
>
> v.rast.stats map=test raster=elevation column_prefix=e
> v.rast.stats map=test_nooverlap raster=elevation column_prefix=e
>
> And here are the results:
>
> > v.db.select test
> cat|id|e_number|e_minimum|e_maximum|e_range|e_average|e_
> stddev|e_variance|e_coeff_var|e_sum|e_first_quartile|e_
> median|e_third_quartile|e_percentile_90
> 1|1|242621|76.8941955566406|145.712738037109|68.8185424804688|114.
> 397437714797|13.5650727983398|184.01120002426|11.8578467047128|27755220.
> 7358017|104.996|115.337|124.654|131.38
> 2|2|131854|77.3364105224609|126.733070373535|49.3966598510742|99.
> 4588066985328|10.6819214497898|114.103445859479|10.7400458585508|
> 13114041.4984283|91.5773|98.7388|106.926|114.645
> 3|3|99039|71.4758224487305|139.941131591797|68.4653091430664|107.
> 223648245541|13.7481896908839|189.012719776527|12.8219752972784|10619322.
> 8985901|97.4993|105.891|118.752|126.703
>
> > v.db.select test_nooverlap
> cat|id|e_number|e_minimum|e_maximum|e_range|e_average|e_
> stddev|e_variance|e_coeff_var|e_sum|e_first_quartile|e_
> median|e_third_quartile|e_percentile_90
> 1|1|242621|76.8941955566406|145.712738037109|68.8185424804688|114.
> 397437714797|13.5650727983398|184.01120002426|11.8578467047128|27755220.
> 7358017|104.996|115.337|124.654|131.38
> 2|2|131854|77.3364105224609|126.733070373535|49.3966598510742|99.
> 4588066985328|10.6819214497898|114.103445859479|10.7400458585508|
> 13114041.4984283|91.5773|98.7388|106.926|114.645
>
> You can see that the values for polygons 1 and 2 are strictly
> identical. Overlapping areas are thus correctly taken into account.
>
> To test even further, I ran
>
> v.extract -d test cat=3 out=test3
>
> to get the entire polygon with cat=3.
>
> Then
>
> v.rast.stats test3 rast=elevation colprefix=e -c
> > v.db.select test3
> cat|id|e_number|e_minimum|e_maximum|e_range|e_average|e_
> stddev|e_variance|e_coeff_var|e_sum|e_first_quartile|e_
> median|e_third_quartile|e_percentile_90
> 3|3|224074|71.4758224487305|141.793502807617|70.
> 3176803588867|110.21428988784|13.2281044492473|174.
> 982747320197|12.0021681968|24696156.7923279|100.265|110.
> 004|120.916|127.836
>
> There thus seems to be a bug in v.rast.stats as it does not calculate
> the correct values for the polygon with cat=3 when there area polygons
> with multiple cat values.
>
> I'll post a bug report.
>
> Moritz
>
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