# Re: [GRASS-user] zonal statistics/metrics - beyond simple statistics

``` Hi Helmut and Stefan,
thanks for sharing that with me, it is good to know that there are similar
ongoingÂ approaches to solve related issues.
Helmut,very nice addon. However, it seems to me that the part on calculating
statistics is still based on v.rast.stats, am I right?
Stefan, the question you pose is indeed very related. I took a better look at
my code and I noticed that what really takes time is not the process of making
a mask, but what comes before: the rasterization of one of the polygons of the
input vector.What I do is:1) g.region using the vector layer, aligning it to
the first of the input rasters2) v.to.rast selecting only one of the polygons
at a time from the vector layer3) r.mask using the rasterized polygon4)
g.region zoom=rasterized_polygon5) calculate any function for all raster maps
(in principle using a loop), using this small region and the mask, and
attaching the results to the attribute table6) repeat 1-5 for all polygons in
the input vector
However, I noticed now that what takes time is v.rast.stats, since it depends
on the whole region selected, which in my case is much bigger than a single
polygon (the vector is a set of ~6,000 small polygons).Do you guys know if
there is a way of using a single polygon from a vector to define the region
using something like g.region?[this would be really great!!]something
likeg.region vector=vector_of_interest cat=1
Best,Bernardo    Em terĂ§a-feira, 10 de abril de 2018 06:01:34 BRT, Stefan
Blumentrath <stefan.blumentr...@nina.no> escreveu:

Hei Bernardo,```
```
Just for the record: I am currently working on something similar / related.

First of all I would like to improve speed of v.rast.stats for multiple inputs:
https://www.mail-archive.com/grass-dev@lists.osgeo.org/msg52562.html and
https://trac.osgeo.org/grass/ticket/3523

Adding a functionality to "tabulate areas" or "count categories" is a next step
I have in mind which I am working on in v.rast.bufferstats (will be a port of
v.what.rast.buffer [1] to GRASS 7 / pygrass (with some enhancements)). Once I
have a good solution there I would like to see if it can be moved to
v.rast.stats. Support for dbf driver is however a challenge...

I will add my "work in progress" script to github, so you can have a look if
you like.

Cheers
Stefan

-----Original Message-----
From: grass-user <grass-user-boun...@lists.osgeo.org> On Behalf Of Helmut
Kudrnovsky
Sent: mandag 9. april 2018 23.19
To: grass-user@lists.osgeo.org
Subject: Re: [GRASS-user] zonal statistics/metrics - beyond simple statistics

Bernardo Santos wrote
> Dear GRASS list,
> I am developing a Python script to be able to calculate (virtually
> any) metrics or statistics for zones/polygons in a vector - in analogy
> to zonal statistics (such as v.rast.stats).The idea is that one can
> calculate raster-based metrics (such as proportion of habitat, number
> of patches, or any metric that can be formalized as a function that
> takes some information from the input raster and returns a value) for
> each polygon in the vector, and this is updated as a value in a newly
> created column in the attribute table of this vector.
> Is there already anything like that (some addon/module) that I am
> missing, just to avoid re-doing something already created?
> If not, what I am doing is to create a loop over all the features in a
> vector, and for each one I zoom and use the polygon to define a mask
> (using r.mask), so that the calculation of the selected metric is
> performed only over that polygon, and this process is repeated.The
> script allows one to calculate metrics/statistics for multiple raster
> maps at once, and to incorporate other function for statistics also.
> It may be found here:https://github.com/LEEClab/GeneralizedZonalStats
>
> For small vectors this works nicely and I believe it has a great
> potential. However, when I try to calculate metrics for a large
> dataset (e.g. the Brazilian map of cities, with almost 6,000 polygons)
> - and that is when the tool would be interesting -, the process of
> creating each mask takes too long (387 steps), and the tool becomes kind of
> useless.
> Then I have two questions:- First, what drives the number of steps
> GRASS takes to create a mask? Why it is very small for some maps but
> very large for others? I quite don't understand that yet.- Do you
> think of a easier or faster way of doing the same thing (instead of using
> v.rast.stats seems to use r.univar and the option 'zones' for doing
> so, but then one gets restricted to the statistics calculated by this module.
> Any help or comment would be very welcome!
> Best,Bernardo Niebuhr
> _______________________________________________
> grass-user mailing list

> grass-user@.osgeo

> https://lists.osgeo.org/mailman/listinfo/grass-user

have look at

where I've coded some DEM derived stats and characteristics for polygones.

-----
best regards
Helmut
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