[R-sig-Geo] Moving window correlation

[Jaime R. Garcia M.]

On Tue, 2009-12-08 at 21:54 +0100, Jaime R. Garcia M. wrote:
> On Tue, 2009-12-08 at 14:32 -0500, Zia Ahmed wrote:
> > I have two grid maps (100 m cell size). I want to a create another  
> > raster gird  of  correlation  coefficient ( r value) of  two maps using  
> > moving windows approach.  Has someone  any idea how to do it in R?
> > 
> > Thanks
> > 
> > Zia
> > 
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> 
> Dear Zia,
> 
> Some time ago I did exactly that and the script was based on a earlier
> post by Prof. Roger Bivand in this list. (Sorry I couldn't find the link
> anymore)
> 
> Suppose "G1" and "G2" are the two grid maps.
> I assume they are "SpatialGridDataFrames" and have exactly the same
> extent and number of columns and rows. Also that you have libraries (sp)
> and (spdep) loaded
> 
> # Define the size of the moving window, for example (7*7) so you have
> enough neighbors for reliable correlation estimation:
> 
> dist = sqrt(2*((100*3)^2))
> 
> # Create a list of neighbours for the distance criteria
> nb <- dnearneigh(coordinates(G1),0, dist) 
> 
> # And then calculate correlations
> v <- sapply(nb, function(i) cor(G1 at data, G2 at data))

# Small correction

v = sapply(nb, function(i) cor(G1 at data[i,], G2 at data[i,]))

#  And in this case the result should be a numeric vector

coordinates(v) = coordinates(G1)
gridded(v) = TRUE
fullgrid(v) = TRUE
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