多边形内的r sf包质心

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。我发现这个问题是在SpatialPolygon中/内部计算质心，但我使用的是sf包。

以下是玩具数据

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

The simple answer is to replace st_centroid with st_point_on_surface. This won't return the true centroid in cases where the centroid is inside the polygon.

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))

Alternatively

If the polygon has a centroid that is inside the polygon, use that, otherwise, find a point within the polygon.

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)) 

上面的功能st_centroid_within_polygon是根据您为sf软件包参考的问题改编的。在这里st_point_on_surface可以找到有关工作方式的更详尽的评论。