r sf包多边形内的质心

Question

我需要向多边形添加标签，并且通常使用质心，但是质心不会落在多边形内。我发现这个问题Calculate Centroid WITHIN / INSIDE a SpatialPolygon但我正在使用 sf 包。

下面是一个玩具数据

rm(list = ls(all = TRUE)) #start with empty workspace

library(sf)
library(tidyverse)
library(ggrepel)

pol <- st_polygon(list(rbind(c(144, 655),c(115, 666)
                         ,c(97, 660),c(86, 640)
                         ,c(83, 610),c(97, 583)
                         ,c(154, 578),c(140, 560)
                         ,c(72, 566),c(59, 600)
                         ,c(65, 634),c(86, 678)
                         ,c(145, 678),c(144, 655)))) %>%
  st_sfc()

a = data.frame(NAME = "A")
st_geometry(a) = pol

a <- a  %>% 
  mutate(lon = map_dbl(geometry, ~st_centroid(.x)[[1]]),
     lat = map_dbl(geometry, ~st_centroid(.x)[[2]]))

ggplot() +
  geom_sf(data = a, fill = "orange") +
  geom_label_repel(data = a, aes(x = lon, y = lat, label = NAME)) 

结果如下

Answer 1

简单的答案是将 st_centroid 替换为 st_point_on_surface。如果质心位于多边形内部，则不会返回真正的质心。

a2 <- a  %>% 
  mutate(lon = map_dbl(geometry, ~st_point_on_surface(.x)[[1]]),
         lat = map_dbl(geometry, ~st_point_on_surface(.x)[[2]]))

ggplot() +
  ggplot2::geom_sf(data = a2, fill = "orange") +
  geom_label_repel(data = a2, aes(x = lon, y = lat, label = NAME))

或者

如果多边形的质心位于多边形内部，则使用该质心，否则，在多边形内找到一个点。

st_centroid_within_poly <- function (poly) {

  # check if centroid is in polygon
  centroid <- poly %>% st_centroid() 
  in_poly <- st_within(centroid, poly, sparse = F)[[1]] 

  # if it is, return that centroid
  if (in_poly) return(centroid) 

  # if not, calculate a point on the surface and return that
  centroid_in_poly <- st_point_on_surface(poly) 
  return(centroid_in_poly)
}

a3 <- a  %>% 
  mutate(lon = map_dbl(geometry, ~st_centroid_within_poly(.x)[[1]]),
         lat = map_dbl(geometry, ~st_centroid_within_poly(.x)[[2]]))

ggplot() +
  ggplot2::geom_sf(data = a3, fill = "orange") +
  geom_label_repel(data = a3, aes(x = lon, y = lat, label = NAME)) 

上面的函数st_centroid_within_polygon改编自question you reference对于 sf 包。对 st_point_on_surface 工作原理的更全面回顾可以是 found here .