Extract center coordinate polygon

Question

I am new to R and I am looking for a little help. I have a polygonized raster image and I want to get the middle coordinate of the image. I have tried gCentroid but there I get a list of multiple coordinates... how do I get just the middle one?

Answer 1

You can convert your raster to polygon then use gCentroid to extract the centroid. Here is an example taken from this answer

library(raster)
library(rgeos)

# example data
x <- raster(system.file("external/test.grd", package="raster"))

### To get the rectangular extent
e <- extent(x)
# coerce to a SpatialPolygons object
p <- as(e, 'SpatialPolygons')  

# calculate the centroid 
c1 <- gCentroid(p)
plot(p)
plot(c1, col='blue', add=TRUE)

### To get a polygon that surrounds cells that are not NA
# make all values the same. Either do
r = x > -Inf
# or alternatively
# r = reclassify(x, cbind(-Inf, Inf, 1))

pp <- rasterToPolygons(r, dissolve=TRUE)

# calculate the centroid 
c2 <- gCentroid(pp)

# look at the results
plot(x)
plot(p,  lwd=5, border='blue',  add=TRUE)
plot(c1, col='blue', add=TRUE)
plot(pp, lwd=3, border='red', add=TRUE)
plot(c2, col='red',  add=TRUE, pch = 17)

Answer 2

To compute the centre point of a raster, take the centre point of the X and Y extents.

x <- raster(system.file("external/test.grd", package="raster"))
c(xmax(x)+xmin(x), ymax(x)+ymin(x))/2

There's no need to convert the raster to a polygon and then run a centroid algorithm.