How to find second highest value and corresponding layer name in a raster stack in R

问题

I have a rasterstack with 12 layers and I would like to extract the 2nd highest value along with its corresponding layer name. I found codes to order my values into 12 new layers in decreasing order:

rs_ord <- calc(inraster, fun=function(X,na.rm) X[order(X,decreasing=T)])

Now, if I could only do the same but return the corresponding layer's name, it would answer it all.

Thanks, Pierre

回答1:

Depending on the dimensions of your rasters, you may be able to use the following, which I'll demonstrate with dummy data in RasterStack s:

library(raster)
s <- stack(replicate(12, raster(matrix(runif(100000), 1000))))

# coerce s to a data.frame
d <- s[]
# return the second-highest value
sort(d, decreasing=TRUE)[2]
# identify the column containing the second-highest value
col(d)[order(d, decreasing=TRUE)[2]]

If the raster's dimensions are too large to use the above approach, you can instead identify the highest two values of each layer in turn, and then work out which layer has the second-highest value:

# return a matrix whose columns contain the top two values per layer
top_two <- sapply(seq_len(nlayers(s)), function(i) {
  sort(s[[i]][], decreasing=TRUE)[1:2]
})
# return the second-highest value
sort(top_two, decreasing=TRUE)[2]
# identify the column containing the second-highest value
col(top_two)[order(top_two, decreasing=TRUE)[2]]

来源：https://stackoverflow.com/questions/33975346/how-to-find-second-highest-value-and-corresponding-layer-name-in-a-raster-stack