R sf ggplot 标记 ID 中列出的多个多边形

Question

假设我想绘制和标记多边形（数据框中每行一个多边形），我可以执行以下操作：

library(sf)
library(ggplot2)
nc <- sf::st_read(system.file("shape/nc.shp", package = "sf"), quiet = TRUE)
nc_3857 <- sf::st_transform(nc, 3857)
ggplot(nc_3857[1:3, ]) +
  geom_sf(aes(fill = AREA)) +
  geom_sf_label(aes(label = NAME))

这里 NAME 仅指 geometry 变量中的一个多边形点列表，它工作正常。

但是，如果我的 geometry 变量代表每一行的多个多边形列表，我将如何标记它们。我想要一个可以解释不同长度列表的通用版本。例如看这个 df:

df <- structure(list(id= structure(1:2, .Label = c("A1", "A2"
), class = "factor"), geometry = structure(list(structure(list(
    list(structure(c(0, 1, 3, 2, 1, 0, 0, 0, 2, 4, 4, 0), .Dim = c(6L, 
    2L)), structure(c(1, 1, 2, 1, 1, 2, 2, 1), .Dim = c(4L, 2L
    ))), list(structure(c(3, 4, 4, 3, 3, 0, 0, 1, 1, 0), .Dim = c(5L, 
    2L)), structure(c(3.3, 3.3, 3.8, 3.8, 3.3, 0.3, 0.8, 0.8, 
    0.3, 0.3), .Dim = c(5L, 2L))), list(structure(c(3, 4, 4, 
    3, 3, 2, 3, 3), .Dim = c(4L, 2L)))), class = c("XY", "MULTIPOLYGON", 
"sfg")), structure(list(list(structure(c(0, 1, 3, 2, 1, 0, 0, 
0, 2, 4, 4, 0), .Dim = c(6L, 2L)), structure(c(1, 1, 2, 1, 1, 
2, 2, 1), .Dim = c(4L, 2L))), list(structure(c(3, 4, 4, 3, 3, 
0, 0, 1, 1, 0), .Dim = c(5L, 2L)), structure(c(3.3, 3.3, 3.8, 
3.8, 3.3, 0.3, 0.8, 0.8, 0.3, 0.3), .Dim = c(5L, 2L)))), class = c("XY", 
"MULTIPOLYGON", "sfg"))), crs = structure(list(epsg = NA_integer_, 
    proj4string = NA_character_), class = "crs"), n_empty = 0L, precision = 0, bbox = structure(c(xmin = 0, 
ymin = 0, xmax = 4, ymax = 4), class = "bbox"), class = c("sfc_MULTIPOLYGON", 
"sfc"))), row.names = c(NA, -2L), class = "data.frame")

对于我的输出，我循环遍历 id 并绘制对应于每个 id 的多边形列表，所以像这样（没有我的循环）：

ggplot() + 
  geom_sf(data = df[df$id=="A1",])

我想一般性地标记每个多边形列表，因此对于每个图我会有 "Fragment 1"、"Fragment 2"...等。就像在我的第一张图片中一样，这取决于有多少片段（下面示例中的 3 个，对于 id = A1）。

看起来很基础但无法理解？

Answer 1

为了绘图，您需要做的是使用 st_cast 将您的 MULTIPOLYGON 几何分割成多个 POLYGON 几何。默认情况下，这将警告您正在跨几何体复制属性（此处为 id），这在这种情况下很好，但可能会导致错误，具体取决于属性（不要复制像面积这样的测量值！）一旦我们每行有一个几何图形，使用 geom_sf_label.

以相同的方式绘制变得容易

library(tidyverse)
library(sf)
#> Linking to GEOS 3.6.1, GDAL 2.1.3, PROJ 4.9.3
df <- structure(list(id = structure(1:2, .Label = c("A1", "A2"), class = "factor"), geometry = structure(list(structure(list(list(structure(c(0, 1, 3, 2, 1, 0, 0, 0, 2, 4, 4, 0), .Dim = c(6L, 2L)), structure(c(1, 1, 2, 1, 1, 2, 2, 1), .Dim = c(4L, 2L))), list(structure(c(3, 4, 4, 3, 3, 0, 0, 1, 1, 0), .Dim = c(5L, 2L)), structure(c(3.3, 3.3, 3.8, 3.8, 3.3, 0.3, 0.8, 0.8, 0.3, 0.3), .Dim = c(5L, 2L))), list(structure(c(3, 4, 4, 3, 3, 2, 3, 3), .Dim = c(4L, 2L)))), class = c("XY", "MULTIPOLYGON", "sfg")), structure(list(list(structure(c(0, 1, 3, 2, 1, 0, 0, 0, 2, 4, 4, 0), .Dim = c(6L, 2L)), structure(c(1, 1, 2, 1, 1, 2, 2, 1), .Dim = c(4L, 2L))), list(structure(c(3, 4, 4, 3, 3, 0, 0, 1, 1, 0), .Dim = c(5L, 2L)), structure(c(3.3, 3.3, 3.8, 3.8, 3.3, 0.3, 0.8, 0.8, 0.3, 0.3), .Dim = c(5L, 2L)))), class = c("XY", "MULTIPOLYGON", "sfg"))), crs = structure(list(epsg = NA_integer_, proj4string = NA_character_), class = "crs"), n_empty = 0L, precision = 0, bbox = structure(c(xmin = 0, ymin = 0, xmax = 4, ymax = 4), class = "bbox"), class = c("sfc_MULTIPOLYGON", "sfc"))), row.names = c(NA, -2L), class = "data.frame")
df %>%
  st_as_sf %>%
  st_cast("POLYGON", group_or_split = TRUE, warn = FALSE) %>%
  ggplot() +
  geom_sf(aes(fill = id)) +
  geom_sf_label(aes(label = id))

在您的示例中，这看起来有点奇怪，因为 A2 多边形与 A1 多边形相同，因此颜色和标签被掩盖了。确实有 3 个 A1 标签被绘制，如果你在 ggplot 调用之前添加一行 filter(id == "A1") 就可以看到：

df %>%
  st_as_sf %>%
  st_cast("POLYGON", group_or_split = TRUE, warn = FALSE) %>%
  filter(id == "A1") %>%
  ggplot() +
  geom_sf(aes(fill = id)) +
  geom_sf_label(aes(label = id))

^{由 reprex package (v0.2.1)}

于 2019-05-13 创建

R sf ggplot 标记 ID 中列出的多个多边形

R sf ggplot labeling multiple polygon that are listed within ID

r

list

ggplot2

sf