distribution of distance for lags (spdep package)
On Tue, 3 Oct 2006, Horacio wrote:
Dear R-geo people, I just discovered the beauty of the spdep package to make spatial analysis of point data easier... However, as I am climbing the learning curve I have a question regarding the estimation of the the lag in the Moran and/or corelogram function. As I understand, if the you generate a neighbor-list using the knearneigh <http://finzi.psych.upenn.edu/R/library/spdep/html/knearneigh.html> function you get a 'knn' class of the points I k distance apart. Then when you calculate the Moran's I (at least that's what I did) you get the Lag in terms of the number of links which in practical purposes is k. so, the first correlation value (od Moran's I) is the obtained by comaring all the sites that have a direct link between them, then, the second lag does it in terms of the ones that are 2 links apart and so on... My question is perhaps very simple... How can I generate a list to plot the distribution of distances for each lag? I thought nbdists could do it, but are not sure how to go about it...
You are on the right track. Here is an example: data(columbus) col.lags <- nblag(col.gal.nb, 4) # to get a list of neighbour lists at increasing lags ldists <- lapply(col.lags, function(x) nbdists(x, coords)) # to get a list of lists of distances ldists1 <- lapply(ldists, unlist) # to flatten the list to contain vectors of distances, as shown by: str(ldists1) # but note that each link is included, both i-j and j-i sapply(ldists1, summary) # for example will give a summary or more complicated: dens <- lapply(ldists1, function(x) density(x, bw=0.1, from=0, to=3.1)) plot(c(0,3.1), c(0,max(sapply(dens, function(x) max(x$y)))), type="n", xlab="distance", ylab="density") lapply(1:4, function(i) lines(dens[[i]]$x, dens[[i]]$y, lty=i)) for a density plot. Thanks for the question, I can see that summary.nb() needs a rewrite which would have made this a bit easier. Roger
thank you Horacio
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Roger Bivand Economic Geography Section, Department of Economics, Norwegian School of Economics and Business Administration, Helleveien 30, N-5045 Bergen, Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43 e-mail: Roger.Bivand at nhh.no