Create voronoi polygon with simple feature in 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.