[R-sig-Geo] Calculating/applying transition matrices from classified imagery?
Jacob van Etten
jacobvanetten at yahoo.com
Tue Apr 12 23:32:10 CEST 2011
This is not an area I am very familiar at all, but I have also seen the use of *adjacent* cells to determine probabilities. Certain types of land use "expand", e.g. new pixels appear at the margin rather than in isolated spots (think about city expansion).
Perhaps gdistance (or Matrix) comes in handy to work with transition matrices...
Jacob.
--- On Tue, 12/4/11, Robert Hijmans <r.hijmans at gmail.com> wrote:
> From: Robert Hijmans <r.hijmans at gmail.com>
> Subject: Re: [R-sig-Geo] Calculating/applying transition matrices from classified imagery?
> To: r-sig-geo at r-project.org
> Date: Tuesday, 12 April, 2011, 22:07
>
> > Does anyone know of a package (or a suggestion on how
> to implement) to
> > calculate, for two classified raster images of the
> same location but
> > different times, the relative probability of
> transitioning from one class
> > to
> > the other? Additionally, once this is figured
> out, how to apply this
> > transition matrix the one of the rasters to predict
> future per-class
> > probabilities? Thanks! Ideally this should
> support arbitrarily large
> > files.
> >
> > --j
>
> Jonathan,
>
> Is the below what you are after? Perhaps there are some
> Markov chain
> implementations that could provide additional things.
>
> Robert
>
>
> library(raster)
> # creating two classified rasters
> s <- r <- raster(nrow=10, ncol=10)
> r[] <- rep(1:5, each=20)
> s[] = round(r[] + rnorm(ncell(r), sd=0.5))
> s[s<1] <- 1
> s[s>5] <- 5
>
> # cross tabulate to get probabilities of transition
> crt <- crosstab(r, s)
> crt <- crt / apply(crt, 1, sum) # probabilities
>
> # function to apply the cross tab probs
> fun <- function(x) {
> classes <- as.numeric(rownames(crt))
> res <- matrix(nrow=length(x),
> ncol=ncol(crt))
> for (i in 1:length(classes)) {
> tr <-
> t(res[x==classes[i], ])
> tr[] <- crt[i,]
> res[x==classes[i], ]
> <- t(tr)
> }
> res
> }
>
> x <- calc(s,fun=fun)
> layerNames(x) <- paste('p( x =', 1:5, ')')
>
> par(mfrow=c(1,2))
> plot(r, main='first')
> plot(s, main='second')
> x11()
> plot(x)
>
>
>
> --
> View this message in context: http://r-sig-geo.2731867.n2.nabble.com/Calculating-applying-transition-matrices-from-classified-imagery-tp6247825p6266539.html
> Sent from the R-sig-geo mailing list archive at
> Nabble.com.
>
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