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
最佳答案
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 .
关于r sf包多边形内的质心,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/52522872/