Create voronoi polygon with simple feature in R - r

Question

I'm not sure if I completely understood the help page to create voronoi polygons.
library(sf)
# function to get polygon from boundary box
bbox_polygon <- function(x) {
bb <- sf::st_bbox(x)
p <- matrix(
c(bb["xmin"], bb["ymin"],
bb["xmin"], bb["ymax"],
bb["xmax"], bb["ymax"],
bb["xmax"], bb["ymin"],
bb["xmin"], bb["ymin"]),
ncol = 2, byrow = T
)
sf::st_polygon(list(p))
}
nc <- st_centroid(st_read(system.file("shape/nc.shp", package="sf")))["BIR79"]
box <- st_sfc(bbox_polygon(nc))
v <- st_voronoi(nc, box)
plot(v)
output
Any idea to fix it?

Answer 1

Using the st_voronoi() example from the sf doc pages as a starting point, it seems that st_voronoi() doesn't work with an sf object consisting of points.
library(sf)
# function to get polygon from boundary box
bbox_polygon <- function(x) {
bb <- sf::st_bbox(x)
p <- matrix(
c(bb["xmin"], bb["ymin"],
bb["xmin"], bb["ymax"],
bb["xmax"], bb["ymax"],
bb["xmax"], bb["ymin"],
bb["xmin"], bb["ymin"]),
ncol = 2, byrow = T
)
sf::st_polygon(list(p))
}
nc <- st_read(system.file("shape/nc.shp", package="sf"))["BIR79"]
nc_centroids <- st_centroid(nc)
box <- st_sfc(bbox_polygon(nc_centroids))
head(nc_centroids)
Each point has a separate geometry entry.
Simple feature collection with 6 features and 1 field
geometry type: POINT
dimension: XY
bbox: xmin: -81.49826 ymin: 36.36145 xmax: -76.0275 ymax: 36.49101
epsg (SRID): 4267
proj4string: +proj=longlat +datum=NAD27 +no_defs
BIR79 geometry
1 1364 POINT(-81.4982613405682 36....
2 542 POINT(-81.125145134236 36.4...
3 3616 POINT(-80.6857465738484 36....
4 830 POINT(-76.0275025784544 36....
5 1606 POINT(-77.4105635619488 36....
6 1838 POINT(-76.9947769754215 36....
This combines the points into a multipoint geometry:
head(st_union(nc_centroids))
Output:
Geometry set for 1 feature
geometry type: MULTIPOINT
dimension: XY
bbox: xmin: -84.05976 ymin: 34.07663 xmax: -75.80982 ymax: 36.49101
epsg (SRID): 4267
proj4string: +proj=longlat +datum=NAD27 +no_defs
MULTIPOINT(-84.0597597853139 35.131067104959, -...
Using the union of points instead of the original sf object works:
v <- st_voronoi(st_union(nc_centroids), box)
plot(v, col = 0)
And here's how to get the correct state boundary instead of the original bounding box.
plot(st_intersection(st_cast(v), st_union(nc)), col = 0) # clip to smaller box
plot(nc_centroids, add = TRUE)
I'm trying to do something similar with marked points and I need to preserve the attributes of the points for the resulting tiles. Haven't figured that out yet.