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我正在创建一个等值线地图并尝试为大都市统计区域绘制边界。我设法完成了大部分工作，但我只想留下外部边界并摆脱该区域内较小区域的线条。下面是可重现的代码行。提前感谢所有的帮助和时间！

library(tigris)
library(sf)
library(tidyverse)
options(tigris_class = "sf")
options(tigris_use_cache = TRUE)

## Getting the Metro & Micro areas
ks <- tracts("KS", cb = T)
cb <- core_based_statistical_areas(cb = T)
str(cb)
# Filter for Kansas metro
kmsa <- cb %>% 
  filter(grepl("KS", NAME)) %>% 
  filter(LSAD == "M1")

ggplot(kmsa) + geom_sf()

# Leave only within KS
w1 <- st_within(ks, kmsa)

w2 <- map_lgl(w1, function(x) {
  if (length(x) == 1) {
    return(TRUE)
  } else {
    return(FALSE)
  }
})

# Subset 
kmsa2 <- ks[w2,]

# Plot
ggplot() + 
  geom_sf(data = ks, fill = "grey50", lwd = 0) +
  geom_sf(data = kmsa2, fill = NA, color = "white") +
  theme_minimal() +
  theme(axis.text.x = element_blank(),
        axis.text.y = element_blank(),
        axis.ticks.x = element_blank(),
        axis.ticks.y = element_blank(),
        panel.grid.major = element_blank())
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