question on raster Moran's I statistical significance
Hello Thank you very much. I have large raster layers and would like to ask, in order to reduce the processing time of the simulation, choosing a smaller nsim value could help ? and if so, what could be the minimum nsim value recommended? Thanks a lot again. Kind regards,
On Wed, Oct 20, 2021 at 8:45 PM Roger Bivand <Roger.Bivand at nhh.no> wrote:
On Wed, 20 Oct 2021, Gabriel Cotlier wrote:
Hello, I would like to estimate the Moran's *I* coefficient for raster data and together with the statical significance of the spatial autocorrelation obtained. I found that the raster package function Moran() although calculates the spatial autocorrelation index it apparently does not give directly the statical significance of the results obtained : https://search.r-project.org/CRAN/refmans/raster/html/autocor.html Could it be be possible to obtain the statistical significance of the results with either raster package or similar one?
fortunes::fortune("This is R")
library(raster)
r <- raster(nrows=10, ncols=10)
values(r) <- 1:ncell(r)
f <- matrix(c(0,1,0,1,0,1,0,1,0), nrow=3)
(rI <- Moran(r, f))
r1 <- r
nsim <- 499
res <- numeric(nsim)
set.seed(1)
for (i in 1:nsim) {
values(r1) <- values(r)[sample(prod(dim(r)))]
res[i] <- Moran(r1, f)
}
Hope-type tests date back to Cliff and Ord; they are permutation
bootstraps.
r_g <- as(r, "SpatialPixelsDataFrame")
library(spdep)
nb <- poly2nb(as(r_g, "SpatialPolygons"), queen=FALSE)
set.seed(1)
o <- moran.mc(r_g$layer, nb2listw(nb, style="B"), nsim=nsim,
return_boot=TRUE)
x_a <- range(c(o$t, o$t0, res, rI))
plot(density(o$t), xlim=x_a)
abline(v=o$t0)
lines(density(res), lty=2)
abline(v=rI, lty=2)
It is not immediately obvious from the code of raster::Moran() why it is
different, possibly because of padding the edges of the raster and thus
increasing the cell count.
For added speed, the bootstrap can be parallelized in both cases; polygon
boundaries are perhaps not ideal.
Hope this clarifies. Always provide a reproducible example, never post
HTML
mail.
Roger Bivand
Thanks a lot.
Kind regards,
Gabriel
[[alternative HTML version deleted]]
_______________________________________________ R-sig-Geo mailing list R-sig-Geo at r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-geo
-- Roger Bivand Emeritus Professor Department of Economics, Norwegian School of Economics, Postboks 3490 Ytre Sandviken, 5045 Bergen, Norway. e-mail: Roger.Bivand at nhh.no https://orcid.org/0000-0003-2392-6140 https://scholar.google.no/citations?user=AWeghB0AAAAJ&hl=en
Gabriel Cotlier, PhD Haifa Research Center for Theoretical Physics and Astrophysics (HCTPA) University of Haifa Israel [[alternative HTML version deleted]]