[R-sig-Geo] select random "blocks" from grid

[Dylan Beaudette]

Hi,

Is there a simple approach to selecting random "blocks" using the sp classes 
and methods?

Say I have a 10x10 grid of polygons, and would like to randomly select 10 of 
these, and save  them to a new sp class (for later saving to GRASS, etc.). Is 
there a simple approach to this?

I have tried something along the lines of accessing the polygons directly:

s <- sample(1:100, size=10)

# doesn't work
grid at polygons[[s]]

# sort of works... but how can I 'upgrade' the result back to a complete set 
# of polygons which can be saved to GRASS?
apply(matrix(s), 1, function(i) grid at polygons[[i]])

Ultimately I would like to establish several randomly placed 100m x 100m 
squares within a square region. Setting up a grid, and selecting random cells 
seemed like a simple approach, but if there is a better one I would be happy 
to hear about it!

I suppose that I could hand-make something that generates random points, 
expands their geometry to 100x100 meters, and then tests to see if the 
resulting rectangle falls completely within the region of interest. This 
would work as a last resort-- surely there must be a cleaner approach within 
the existing framework (I hope!). 

Cheers,

Dylan



-- 
Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341