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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