R sf::st_convex_hull() 在多组数据上丢失 sf 类

Question

我正在尝试在 R 中使用 sf 创建一个 95% 的最小凸多边形。只要我只将数据分组到 1 个变量上，我的代码就可以正常工作，但是当我分组到两个变量上时，输出将失去其 sf 类并且改为 grouped_df。

这里以一个玩具数据集为例

library(dplyr) 
set.seed(12)
toy <- tibble::tibble(
    ID = rep(c(1,2), each = 10),
    year = rep(c(1,2), 10),
    lat = runif(20, 1, 10),
    long = runif(20, 1, 10)
  ) %>%
    sf::st_as_sf(., coords = c("long", "lat")) 
toy %>%
  group_by(ID) %>% 
  summarize(.groups = "keep") %>% 
  mutate(cent = sf::st_centroid(geometry)) %>%
  sf::st_cast(to = "POINT") %>%  
  mutate(dist = sf::st_distance(geometry, cent, by_element = TRUE)) %>% 
  filter(dist <= quantile(dist, .95)) %>% 
  summarize()  %>%
  sf::st_convex_hull() %>%
  class()

这给出了我想要的输出。但是当我尝试按两个变量分组时，结果丢失了 sf 类。

toy %>%
  group_by(ID, year) %>% 
  summarize(.groups = "keep") %>% 
  mutate(cent = sf::st_centroid(geometry)) %>%
  sf::st_cast(to = "POINT") %>%  
  mutate(dist = sf::st_distance(geometry, cent, by_element = TRUE)) %>% 
  filter(dist <= quantile(dist, .95)) %>% 
  summarize()  %>%
  sf::st_convex_hull() %>%
  class

我的代码中是否有什么东西阻止我对两个变量进行分组？

Answer 1

因为第二个summarize在ID上重新分组。您需要一个 .groups = "keeps" 来传递相同的分组，然后转换回 sf 对象。或者，您可以创建一个分组变量 mutate(grp = paste0(ID, year)) 并执行 group_by(grp) 。

toy %>%
  group_by(ID, year) %>% 
  summarize(.groups = "keep") %>% 
  mutate(cent = sf::st_centroid(geometry)) %>%
  sf::st_cast(to = "POINT") %>%  
  mutate(dist = sf::st_distance(geometry, cent, by_element = TRUE)) %>% 
  filter(dist <= quantile(dist, .95)) %>% 
  summarize(.groups = "keep")  %>%
  sf::st_convex_hull() %>%
  st_sf()