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rraster

Find 8 neighbors of a point in a raster in R?

If I have a raster like this:

 library(raster)
 r1 <- raster(ncol=10, nrow=10)
 r1[] <- runif(ncell(r1))
 r1[] = 1:ncell(r1)
 x=c(-6)            
 y=c(-67)

I can extract values that correspond to this point as:

values = extract(r1, SpatialPoints(cbind(y,x),CRS("+init=EPSG:4326")))

However, this extracts the closest pixel to the coordinates of this point. I need to extract the 9 closest pixels (8 neighbors to the pixel extracted)to this point. Any idea?

Solution

  • You can use the adjacent method for that.

    Example data

    library(raster)
r <- raster(ncol=10, nrow=10)
values(r) = 1:ncell(r)
xy <- cbind(-67, -6)

    Solution

    cells <- cellFromXY(r, xy)
adj <- adjacent(r, cells, 8, include=TRUE)

r[adj[,2]]
# [1] 54 43 53 63 45 55 65 44 64

    This returns the value for the cells (because include=TRUE) and their 8 neighbors