[GRASS-user] Create random raster map with sum of cells == N

[Rainer M Krug]

Hi Johannes,

thanks a lot for the info. I am doing effectively the same now by using
R and rbinom but I am switching to rmultinom..

Cheers,

Rainer

Johannes Radinger <johannesradinger at gmail.com> writes:

> Hi Rainer,
>
> I don't know of any standard GRASS tool that performs such tasks.
>
> However I had a similar task, also an ecological topic and it was about
> the redistribution of fish according to a dispersal kernel. So in my case
> a number of fish (n_individuals) should be randomly distributed over a
> raster map, but in my case with varying probabilities (used as weighting
> factor) in the landscape.
>
> I solved that with the python/array interface of GRASS, in detail with a
> random multinomial approach of numpy:
>
> # Import Python modules
> import grass.script.array as garray
> import numpy
>
> # Probability map, varying probability over the landscape, used as
> weighting factor
> probmap = garray.array()
> probmap.read("probmap")
>
> # Output, redistributed individuals (n=500)
> outputmap = garray.array()
> n_individuals=500
>
> outputmap[...] = numpy.random.multinomial(n_individuals,
> (probmap/numpy.sum(probmap)).flat, size=1).reshape(probmap.shape)
> outputmap.write("outputmap")
>
>
> This approach is also implemented in the newest version of the GRASS-GIS
> addon r.fidimo (GRASS70), which
> is a fish dispersal model for river networks.
>
> So maybe that approach helps you!
>
> Best,
>
> Johannes
>
>
>
>
> On Fri, Sep 6, 2013 at 1:17 PM, Rainer M Krug <Rainer at krugs.de> wrote:
>
>> Hi
>>
>> I am writing a simulation of dispersal, and one possible dispersal
>> approach is to distribute a certain number of seeds N randomly in the
>> landscape.
>>
>> Therefore I would like to generate a random raster map with the sum of
>> all cells equals to N. Is there a way of doing this easily in GRASS?
>>
>> At the moment, I am using an R function, which distributes the seeds,
>> but due to computational time considerations, I would like to move it to
>> GRASS.
>>
>> Thanks,
>>
>> Rainer
>>
>>
>> --
>> Rainer M. Krug
>>
>> email: RMKrug<at>gmail<dot>com
>>
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-- 
Rainer M. Krug, PhD (Conservation Ecology, SUN), MSc (Conservation Biology, UCT), Dipl. Phys. (Germany)

Centre of Excellence for Invasion Biology
Stellenbosch University
South Africa

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