Selecting all polygons along northern edge of shapefile in R

Question

I have a shapefile of Mexico composed of Admin 2 polygons.

I am trying to create a buffer along the Mexico-US border, in R considering this is a shapefile of Mexico only and I want to retain the admin 2 boundaries after the buffer is created.

So I am trying to find a way of selecting all the bordering admin 2 polygons. The admin 2 polygons are of varying size with variable numbers of neighbours.

---

@Spacedman provided feedback -- I found this resource, which was helpful, and my code now looks as follows:

#subset to mexico and us only
mexus <- adm0all[adm0all$ADM0_NAME %in% c('Mexico', 'United States of 
America'),]

#identify shared border
sf_bord <- mexus %>% 
            ms_innerlines() %>% 
            as_tibble() %>% 
            st_as_sf()

#plot & see if worked
ggplot()+
  geom_sf(data=mexus) +
  geom_sf(data=sf_bord, color = "red", size = 1) +
  coord_sf(datum = NA) +
  theme_void()

This results in the border being isolated(see image above), however when I calculate the border using the following code, my buffer is fragmented/non-continuous, see code and image below:

#transform to project CRS to get units in meters
sf_bord_utm <- st_transform(sf_bord, 6368)

#calculate 5km buffer
mexus_border <- st_buffer(sf_bord_utm, dist = 5000)

Does anyone understand why this is not a continuous buffer?

Answer 1

I adopted the following strategy to solve this issue:

• Found a polylines shapefile representing the Mexico-US border from the U.S. Geological Survey website (found here for direct download)
• Brought this into my R script and calculated the 5 km buffer
• Set the CRS to match that of my Mexico Admin0 shapefile
• Used st_intersection to clip buffer to Mexico side only

Code:

#calculate 5km buffer
mexus_border <- st_buffer(mex_us_bord, dist = 5000)

#plot buffer under border over mexico to check worked
ggplot() +
  geom_sf(data=mex) +
  geom_sf(data=mexus_border, color = "red", size = 1) +
  geom_sf(data=mex_us_bord, color = "blue", size = 1) +
  coord_sf(datum = NA) +
  theme_void()

#set buffer area crs to same as mexico
mexus_bord_proj <- st_transform(mexus_border, 4326)

#plot mexico adm0 on top and clip buffer to just mexico side
mex_buff <- st_intersection(mex, mexus_bord_proj)

Thank you to @Spacedman for your help.