Find overlapping polygons after buffer

overlapping_buffers.R

library(sf)
library(ggplot2)
library(dplyr)

# Get data
extraWD <- "."
if (!file.exists(file.path(extraWD, "departement.zip"))) {
  githubURL <- "https://github.com/statnmap/blog_tips/raw/master/2018-07-14-introduction-to-mapping-with-sf-and-co/data/departement.zip"
  download.file(githubURL, file.path(extraWD, "departement.zip"))
  unzip(file.path(extraWD, "departement.zip"), exdir = extraWD)
}
departements_L93 <- st_read(dsn = extraWD, layer = "DEPARTEMENT",
                            quiet = TRUE) %>% 
  st_transform(2154)

# Reduce dataset
dep_bretagne <- departements_L93 %>%
  filter(NOM_REG == "BRETAGNE")

# Create buffer
dept_buf <- dep_bretagne %>% 
  st_buffer(
  dist = 
    units::set_units(30, km)
) 

ggplot(dept_buf) +
  geom_sf(fill = NA)

dept_buf_overlap <- dept_buf %>% 
  st_intersection() %>% 
  st_collection_extract("POLYGON")

ggplot(dept_buf_overlap) + 
  geom_sf(aes(fill = n.overlaps))

I see the problem

Find out which one overlap:

I tried Voronoi over the buffer, but that may be a little more complicated....

dept_voronoi <- dept_buf_overlap %>% 
  st_voronoi()

ggplot() + 
  geom_sf(data = dep_bretagne[1,]) +
  geom_sf(data = dept_voronoi[1,], fill = NA)