Select map region in R

Here is an example of selecting a region from a map by specifying a rectangle for the selection.

Code

cities <-
  maps::world.cities |>
  filter(country.etc == "Germany") |>
  st_as_sf(coords = c("long", "lat"), crs = 4326, agr = "constant") |>
  filter(pop >= 100000) |>
  mutate(label = if_else(pop > 800000, name, NA_character_))

write_rds(cities, "cities-deu.rds")

First, I create a map of German cities with more than 100,000 inhabitants.

Code

library(tidyverse)
library(sf)

cities <- read_rds("cities-deu.rds")

ggplot(cities) +
  geom_sf(aes(size = pop), color = "grey80", alpha = 0.5) +
  geom_sf_label(aes(label = label), alpha = 0.4) +
  guides(size = "none") +
  theme_void()

Then, I select the wider Ruhr area by defining the bottom left and the top right of the selection area.

Code

create_sf_rectangle <- function(xmin, xmax, ymin, ymax, crs = "OGC:CRS84") {
  matrix(
    c(xmin, ymin, xmax, ymin, xmax, ymax, xmin, ymax, xmin, ymin),
    ncol = 2, byrow = TRUE
  ) |>
    list() |>
    st_polygon() |>
    st_sfc(crs = crs)
}

nrw_select <- create_sf_rectangle(6.5, 7.9, 50.9, 51.7)

nrw <-
  cities |>
  st_intersection(nrw_select)

Finally, I create a map of the cities in the wider Ruhr area.

Code

nrw |>
  mutate(label = if_else(pop > 500000, name, NA_character_)) |>
  ggplot() +
  geom_sf(aes(size = pop), color = "grey80", alpha = 0.5) +
  geom_sf_label(aes(label = label), alpha = 0.4) +
  guides(size = "none") +
  theme_void()