question on raster Moran's I statistical significance

On Sat, 23 Oct 2021, Gabriel Cotlier wrote:

Hello,

I would like to estimate the pseudo p-value of Moran's I from raster data
layer, by converting it to polygon data in order to be able to directly

use

the function  spdep::mc.moran() and getting in one step not only value of
Moran'sI but its statistical significance without using other simulations
but only that in the funcion spdep::mc.moran() as an alternative I have
used the code below. However, since with the initial version of the code

I

got  several errors such as :

Error in moran. mc(My_raster, lw.l, 999) : r is not a numeric vector

So I converted "r" to a vector using as.vector() in :

M <- moran.mc(as.vector(r), lw.l, 999)

First, if you have the spatial weights object lw.l, spdep::moran.test(),
using analytical rather than bootstrap inference, is almost certainly more
efficient, and yields almost always the same outcome with
randomisation=TRUE.

However then appeared another the error:

Error in na.fail.default(x) : missing values in object

Please see the na.action= argument to moran.mc() and moran.test(); the
default is na.fail, but can be na.exclude, which then also subsets the
listw spatial weights object. You can also subset l before creating the
neighbour object:

library(raster)
library(spdep)
r <- raster(nrows=10, ncols=10)
values(r) <- c(1:(ncell(r)-1), NA)
l <- rasterToPolygons(r, na.rm=TRUE) # note that this subsets by default
dim(l) # [1] 99  1
nb.l <- poly2nb(l, queen=FALSE)
nb.l # here no no-neighbour observations, no need for zero.policy=
lw.l <- nb2listw(nb.l, style="W")
moran.test(l$layer, lw.l) # default randomisation=TRUE

Finally these errors disappeared with the following version of the code--
see below -- the version of the code below runs OK--unless for the input
raster I have used-- with no more errors or problems.

Because rasterToPolygons() removes NA observations by default.

Hope this clarifies,

Roger

Therefore my question is : if instead of using arguments of the function
spdep::mc.moran() such as na.action = na.omit or other --which I have

tried

and could not make work-- is it a valid way to solve the above errors  in
the way as presented in the code below?

I would appreciate it a lot if there is a possibility of knowing what

could

be wrong in the code below?

I suspect it could be possible the problem is related to the way in which
the raster  data layer is converted to polygon data  im the code to be
input to the function spdep::mc.moran() , but will appreciate any

guidance

and assistance in the issue to get working in the correct manner.

Thanks a lot.
Kind regards,

################### Estimate of Moran's I and its p-value from a raster
layer ##################################
library(raster)
library(spdep)
library(easycsv)

rm(list = ls())

r<- raster(file.choose())
l <- rasterToPolygons(r)

nb.l <- poly2nb(l, queen=FALSE)
lw.l <- nb2listw(nb.l, style="W",zero.policy=TRUE)

v<-r[!is.na(r)]
M <- moran.mc((v), lw.l, 999, zero.policy=TRUE)
M

plot(M, main=sprintf("Original, Moran's I (p = %0.3f)", M$p.value),
xlab="Monte-Carlo Simulation of Moran I", sub =NA, cex.lab=1.5,
cex.axis=1.5, cex.main=1.5, cex.sub=1.5)
abline(v=M$statistic, col="red", lw=2)
dev.off()

################################################################################################

--
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