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

GRASS GRASS user
1

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

2

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@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: RMKruggmailcom

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grass-user@lists.osgeo.org
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3

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@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@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

Tel : +33 - (0)9 53 10 27 44
Cell: +33 - (0)6 85 62 59 98
Fax : +33 - (0)9 58 10 27 44

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email: Rainer@krugs.de

Skype: RMkrug