given is a raster as well as a SpatialPolygonsDataframe. In order to retrieve the highest value of the raster within the area of a polygon, raster::extract can be used. It works fine.
How to get additionally the coordinates of the extracted highest value of the raster within the area of a polygon?
# create raster
r <- raster(ncol=36, nrow=18)
r[] <- runif(ncell(r))
# create SpatialPolygons from GridTopology
grd <- GridTopology(c(-150, -50), c(40, 40), c(8, 3))
Spol <- as(grd, "SpatialPolygons")
# create SpatialPolygonsDataFrame
centroids <- coordinates(Spol)
x <- centroids[,1]
y <- centroids[,2]
SPDF <- SpatialPolygonsDataFrame(Spol, data=data.frame(x=x, y=y, row.names=row.names(Spol)))
# extract max value of raster for each SpatialPolygon
ext <- raster::extract(r, SPDF, fun=max)
*example code is taken from R-documentation
You can use the cellnumbers=TRUE argument in extract. With "terra" you can do
library(terra)
r <- rast(system.file("ex/elev.tif", package="terra"))
v <- vect(system.file("ex/lux.shp", package="terra"))
e <- terra::extract(r, v, xy=TRUE, ID=TRUE)
m <- merge(aggregate(elevation~ID, max, data=e), e)
head(m)
# ID elevation x y
#1 1 547 6.020833 50.17917
#2 10 432 5.987500 49.46250
#3 11 427 6.204167 49.67917
#4 12 413 6.154167 49.77083
#5 2 514 6.087500 49.97917
#6 3 517 5.879167 49.84583
Another way to do the last step
b <- by(e, e$ID, FUN=function(d) d[which.max(d$elevation),])
b <- do.call(rbind, b)
head(b)
# ID elevation x y
#1 1 547 6.020833 50.17917
#2 2 514 6.087500 49.97917
#3 3 517 5.879167 49.84583
#4 4 520 6.112500 49.97917
#5 5 511 5.937500 49.87917
#6 6 403 6.270833 49.83750
And here with your example data and the older packages:
ext <- raster::extract(r, SPDF, cellnumbers=TRUE)
v <- t(sapply(ext, function(i) i[which.max(i[,2]), ] ))
# cell value
# [1,] 185 0.9303460
# [2,] 188 0.9821190
# [3,] 154 0.9926290
# [4,] 232 0.8907819
# [5,] 234 0.9998510
To get the coordinates:
xyFromCell(r, v[,1])
# x y
# [1,] -135 35
# [2,] -105 35
# [3,] -85 45
# [4,] -25 25
# [5,] -5 25