r sf 包质心在多边形内

Question

我需要为多边形添加标签，我通常使用质心，但是质心不会落在多边形内。我发现了这个问题Calculate Centroid WITHIN / INSIDE a SpatialPolygon，但我使用的是 sf 包。

下面是玩具数据

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。如果质心在多边形内，这将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

如果多边形的质心在多边形内，则使用它，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)) 

st_centroid_within_polygon 上面的函数改编自sf 包的question you reference。 found here 可以更全面地了解 st_point_on_surface 的工作原理。

Answer 2

扩展 Mitch 的答案，因为上面提供的 st_centroid_within_poly 函数仅适用于单个多边形。

要在多个多边形上使用，请使用：

st_centroid_within_poly <- function (poly) {

  # check if centroid is in polygon
  ctrd <- st_centroid(poly, of_largest_polygon = TRUE)
  in_poly <- diag(st_within(ctrd, poly, sparse = F))

  # replace geometries that are not within polygon with st_point_on_surface()
  st_geometry(ctrd[!in_poly,]) <- st_geometry(st_point_on_surface(poly[!in_poly,]))

  ctrd
}