[GRASS-user] randomize the position of a raster map

[Milton Ribeiro]

Hi Moritz,

I like your tips.. I will try this.

Thanks!

milton

2014-12-09 12:55 GMT-02:00 Moritz Lennert <mlennert at club.worldonline.be>:

> On 08/12/14 18:26, Milton Ribeiro wrote:
>
>> Hi all,
>>
>> I have a landcover maps, and I need to randomize the position of them,
>> keeping all the pixels on the new raster map.
>>
>
> IIUC, you actually want to randomize the categories of pixels where
> categories are land covers and there is a constraint that you want to keep
> the same number of pixels per category. Is that correct ?
>
>  Is there a command that can do this within GRASS?
>>
>
> You could use r.random following a procedure something like this:
>
> create a raster (mymask) where all cells are non-null
>
> for each category of land cover,
>         get cat and percentage of cover of that cat (e.g. via r.stats)
>         run r.random, using cover=mymask and n=percentage of cover
>         modify mymask so that the cells attributed in the previous run of
> r.random get set to null to make them unavailable in the next run
>
> There is just one issue: the cover option in r.random provokes a check if
> the randomly selected cell is in the cover area or not. If not, it is not
> used, but no replacement is found, so if you use cover= the percentage
> value in n= is not respected... Ideally this should be corrected to while
> cell number X is not within the cover, find a new cell. See [1] for an old
> discussion about this.
>
> Attached is a bash script that tries to work around this issue and works
> more or less for me (at least with the "landuse dataset in the simplified
> NC dataset). Certainly not perfect, but maybe you can build on that.
>
> Another option might be something like this:
>
> Knowing that:
>
> r.stats -ncp landuse --quiet
> 1 73077 29.31%
> 2 2137 0.86%
> 3 25518 10.24%
> 4 16829 6.75%
> 5 126265 50.64%
> 6 5303 2.13%
> 7 194 0.08%
>
> use the random generator in r.mapcalc
>
> r.mapcalc "eval(random = rand(0.0,1.0)); landuse_randomized_mapcalc =
> if(random<=0.2931,1, if(random>0.2931 && random <=0.3017,2,if(random>0.3017
> && random <=0.4041, 3, if(random>0.4041 && random<=0.4716, 4, if(random >
> 0.4716 && random <=0.9780, 5, if(random>0.9780 && random <=0.9993, 5,
> 6))))))"  --o
>
> The result is not perfect in terms of cell counts, but close:
>
> r.stats -ncp landuse_randomized_mapcalc --quiet
> 1 73402 29.46%
> 2 2061 0.83%
> 3 25215 10.12%
> 4 16982 6.82%
> 5 131484 52.78%
> 6 180 0.07%
>
>
> Moritz
>
> [1] https://trac.osgeo.org/grass/ticket/1082
>
>
>
>
>


-- 
Miltinho - mcr at rc.unesp.br
Laboratório de Ecologia Espacial e Conservação - LEEC
Depto de Ecologia - UNESP - Rio Claro
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