Rasterizing Spdep's localG output

I'm reasonably new to R and I am trying to rasterize the output of spdep's localG function. This code:

neigh2<-dnearneigh(profcurvPts, 0, 2)
list<-nb2listw(neigh2)
gistar<-localG(profcurvPts$layer, list)
girast<-rasterize(gistar, profcurv)

Yields an error unable to find an inherited method for function 'rasterize' for signature '"localG", "RasterLayer"'

I have tried changing the localG class to a data.frame, but it creates a 1 column matrix that still won't rasterize.

To sum it up: what should I do to get a raster of the localG output?

Thanks in advance!

Solution

You are trying to call an object of class localG which has no associated method for sp or raster classes. Here is a workflow for rasterizing a local G result.

First, add packages and data. The meuse object is of SpatialPointsDataFrame and meuse.grid starts as SpatialGridDataFrame but is coerced into a rasterLayer object for rasterizing the point data.

library(spdep)
library(raster)

data(meuse)
  coordinates(meuse) <- ~x+y
  proj4string(meuse) <- CRS("+init=epsg:28992")  

data(meuse.grid)
  coordinates(meuse.grid) = ~x+y
  proj4string(meuse.grid) <- CRS("+init=epsg:28992")
  gridded(meuse.grid) = TRUE
  meuse.grid <- raster(meuse.grid)

Here we conduct the local G analysis.

nb <- dnearneigh(coordinates(meuse), 0, 500)
G <- localG(meuse$cadmium, nb2listw(nb, style="B"))

This is where we can coerce the localG results, join them to the point data and rasterize the results. You can use as.numeric for coercing from a localG object (basically a list object). Please read the help for raster::rasterize. The x argument requires SpatialPoints or a matrix of coordinates, y is a rasterLayer object to provide the raster dimensions and field represents the attribute that is being rasterized. If you need a background value for the raster that is other than NA then use the background argument.

meuse$G <- as.numeric(G)
spplot(meuse, "G")

Gr <- rasterize(coordinates(meuse), meuse.grid, field = meuse$G, background = NA)