How can I change the color of a shared border using sf?

I would like to change the color of a shared color to a different color, let’s say red. So far, I am plotting the German federal state Bavaria and touching Austrian states. I get the data from https://gadm.org/download_country.html –

Germany Level 2 – https://biogeo.ucdavis.edu/data/gadm3.6/Rsf/gadm36_DEU_2_sf.rds

Germany Level 1 – https://biogeo.ucdavis.edu/data/gadm3.6/Rsf/gadm36_DEU_1_sf.rds

Austria Level 2 – https://biogeo.ucdavis.edu/data/gadm3.6/Rsf/gadm36_AUT_2_sf.rds

Austria Level 1 –
https://biogeo.ucdavis.edu/data/gadm3.6/Rsf/gadm36_AUT_1_sf.rds

library("sf")
library("raster")
library("dplyr")
library("spData")
library("spDataLarge")
library("ggplot2")
library("patchwork")
library(tmap)    # for static and interactive maps
library(ggpattern)

data_aut <- readRDS("~/plot_at_ger/data/gadm36_AUT_2_sf.rds")
data_ger <- readRDS("~/plot_at_ger/data/gadm36_DEU_2_sf.rds")
data_aut_high <- readRDS("~/plot_at_ger/data/gadm36_AUT_1_sf.rds")
data_aut_high <- data_aut_high[which(data_aut_high$NAME_1=='Salzburg' | data_aut_high$NAME_1=='Oberösterreich' | data_aut_high$NAME_1=='Tirol' | data_aut_high$NAME_1=='Vorarlberg'), ]
data_ger_high <- readRDS("~/plot_at_ger/data/gadm36_DEU_1_sf.rds")
data_ger_high <- data_ger_high[which(data_ger_high$NAME_1=='Bayern'), ]

ggplot() +
  geom_sf(data = ger_selected_data_bavaria, fill = NA) +
  geom_sf(data = aut_selected_data_rel, fill = NA) +
  geom_sf(data = data_aut_high, fill = NA, size = 1, color = "grey35") +
  geom_sf(data = data_ger_high, fill = NA, size = 1, color = "black") 

This produces the following figure:

Is there a way to change the color of the shared border?

Thanks!

>Solution :

Drawing a shared border is kind of tricky in context of {sf} > 1.0, as it uses the s2 dependency for spherical operations, and the s2 library introduced a new concept of semi-closed polygons (which was not there in the GEOS years).

See https://r-spatial.github.io/s2/reference/s2_options.html#model for more information.

Anyhow, consider this piece of code, built on {giscoR} package to access EU NUTS regions, and sf::st_intersection() to find the shared border. Note the use of model = "closed" (i.e. all polygons contain their entire boundaries) which may not be immediately obvious, and is necessary for the code to work as intended.

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

bavaria <- gisco_get_nuts(country = "DE",
                          nuts_level = "1") %>% 
  filter(NUTS_NAME == "BAYERN")

austria <- gisco_get_nuts(country = "AT",
                          nuts_level = "2")

shared_border <- st_intersection(bavaria,
                                 austria,
                                 model = "closed") # this line is important!

ggplot() +
  geom_sf(data = bavaria, fill = NA, color = "gray40") +
  geom_sf(data = austria, fill = NA, color = "gray40") +
  geom_sf(data = shared_border, fill = NA, color = "red")