[GRASS-user] Select random cells from map

[Hamish]

> > 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