[GRASS-user] Select random cells from map

[António Rocha]

Thanks Hamish
I thought that random cells are retrieved from valid "map" cells. Or not?
Antonio

Hamish wrote:
>>> António wrote:
>>>       
>>>> I need to randomly select non-null cells from a given map.
>>>> I tried r.random.cells but I cannot define a maximum number
>>>> random cells and some selected random cells are not valid
>>>> values in my map. is there any other function to randomly
>>>> select cells from a raster?
>>>>         
>
> Hamish:
>   
>>> r.to.vect to get a vector area coverage of your raster (do
>>> not use the -s flag), then v.random.cover from wiki addons
>>> then v.to.rast, and finally r.mapcalc to transfer values from
>>> source map to random cell map? (probably we/I should make a
>>> new wrapper script to do that automatically)
>>>
>>> or you might skip the last steps and just use v.random.cover's
>>> raster= option to sample the raster map directly.
>>>
>>> probably the best idea is to fix r.random's cover= option to
>>> loop until it has enough points.
>>>
>>>
>>> also if you will be doing a lot of geostats it is probably
>>> worth your while to learn about the GRASS<->R interface:
>>>  http://grass.osgeo.org/wiki/R_statistics
>>>
>>> ... then the world is open to you.
>>>       
>
> António  wrote:
>   
>> Thanks for your answer. I tried r.random with my input data and the 
>> number of cells and it worked but I do have one question: I didn't 
>> understand the objective of cover_map. I used the map from I want to 
>> extract random cells as input but what is the objective of cover_map?
>>     
>
> r.random creates random points all over the current rectangular region
> block.
>
> Adding a cover map makes those random points only occur within the
> irregular area covered by the cover map. (but r.random cover= is a bit
> buggy right now; see new ticket in the bug trac'er)
>
>
> Hamish
>
>
>
>       
>
>
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