获取美国和加拿大各县的中心经纬度

Question

我能够获得美国每个县和加拿大人口普查地理单位（CGU）的中心纬度和经度。然而，在这个过程中出现了一些虚假的县，特别是在五大湖地区。在该地区，五大湖（州/国际边界？）内有一些随机多边形产生这些虚假县。有没有办法删除这些水生多边形，以便我只获得陆地多边形的中心纬度和经度？

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

# Gets the polygons of each US county and Canadian CGU

can <- gadm(country = 'CAN', level = 2, path=tempdir()) %>%
  st_as_sf()

usa <- gadm(country = 'USA', level = 2, path=tempdir()) %>%
  st_as_sf()

# Gets the central latitude and longitude of each county and CGU

centroids_can <- can %>% 
  st_centroid(of_largest_polygon = TRUE)

centroids_usa <- usa %>% 
  st_centroid(of_largest_polygon = TRUE)

# Map of centroids

ggplot() +
  geom_sf(data = can,
          mapping = aes(geometry = geometry),
          color = "black") +
  geom_sf(data = usa,
          mapping = aes(geometry = geometry),
          color = "black") +
  geom_sf(data = centroids_can,
          mapping = aes(geometry = geometry),
          color = "black",
          fill = 'red',
          shape = 21,
          size = 2) +
  geom_sf(data = centroids_usa,
          mapping = aes(geometry = geometry),
          color = "black",
          fill = 'red',
          shape = 21,
          size = 2) +
  coord_sf(xlim = c(-93, -81.5), ylim = c(41.5, 50), expand = F) +
  theme_bw() +
  theme(plot.margin = grid::unit(c(2,2,0,2), "mm"),
        text = element_text(size = 16),
        axis.text.x = element_text(size = 14, color = "black"),
        axis.text.y = element_text(size = 14, color = "black"),
        panel.grid.major = element_blank())

明显不正确的县和 CGU 是黄色的，红色的看起来是正确的。我说很明显，因为五大湖的水生区域没有县或 CGU。