菊花链通过单个最近点连接多个多边形

Question

我希望为多个多边形创建一个遮罩多边形。

单个多边形很容易：

How to apply a polygon mask layer in ggplot

但对于多个来说要复杂得多：

https://www.stat.auckland.ac.nz/~paul/Reports/GraphicsEngine/definitions/definitions.html

我觉得我已经很接近了，但我需要一种在最近多边形的点之间画一条线的方法，即我想尝试用一条线连接最近的岛屿。

同时将这些多边形连接到外部多边形框，但仅使用一条线：

library(tidyverse)
library(sf)
library(albersusa)

usa <- usa_sf()

HI <- st_coordinates(usa %>%
                             filter(name %in% c("Hawaii"))) %>%
        as.data.frame() %>%
        select(X, Y)


rec_box <-
        data.frame(
                X = c(-108,-108,-101,-101,-108),
                Y = c(24, 28, 28, 24, 24)
        )

mask <- rbind(HI, rec_box)

eg <- st_as_sf(data.frame(mask), coords = c("X", "Y"))
poly <- st_convex_hull(eg)

ggplot() +
        geom_sf(data = poly) +
        geom_density2d_filled(data = HI, aes(x = X, y = Y)) +
        geom_polygon(data = mask,
                     aes(x = X, y = Y),
                     color = "black",
                     fill = "white")

Answer 1

编辑： 仅用于岛屿周围的面具：

library(tidyverse)
library(sf)
#> Linking to GEOS 3.8.0, GDAL 3.0.4, PROJ 6.3.1
library(albersusa)
#library(concaveman)
#library(nngeo)

usa <- usa_sf()

# keep only the geometry from Hawaii (not the data)
HI <- usa %>%
       filter(name %in% c("Hawaii")) %>%
       st_cast('POLYGON') %>% 
       st_geometry() %>% 
       st_as_sf()


# HI as dataframe as your example
HI2 <- st_coordinates(usa %>%
        filter(name %in% c("Hawaii"))) %>%
        as.data.frame() %>%
        select(X, Y)
# you may need a larger box. This uses sf:st_bbox to get exact
#  rectangle of the polygons. adjust accordingly
hi_box <- st_bbox(HI) %>% st_as_sfc() %>% st_as_sf()

# find the difference betwee HI & hi_box
hi_mask <- st_difference(hi_box, st_union(HI))


ggplot() +
  geom_density_2d_filled(data = HI2, aes(x = X, y = Y)) +
  geom_sf(data = hi_mask, fill = 'black') 

^{由reprex package (v0.3.0) 于 2021 年 4 月 4 日创建}

如果您对上面创建的 HI 掩码感到满意，可以使用带有 nngeo::st_connect() 的单行将其连接到您定义的边界框。

这应该找到两者之间最短的连接线。如果您希望连接线位于特定点（上例中的西南角），请将其作为 sf 对象提供给 st_connect() 函数，而不是整个框。

library(tidyverse)
library(sf)
#> Linking to GEOS 3.8.0, GDAL 3.0.4, PROJ 6.3.1
library(albersusa)
library(concaveman)
library(nngeo)

usa <- usa_sf()

# Hawaii, cast to POLYGON rather than the original MULTIPOLYGON
HI <- (usa %>%
       filter(name %in% c("Hawaii"))) %>%
       st_cast('POLYGON')

connected_HI <- concaveman(HI)

# rec box as an sf object with same crs as HI
rec_box <-
  data.frame(X = c(-108, -108, -101, -101, -108),
             Y = c(24, 28, 28, 24, 24)) %>%
  st_as_sf(coords = c('X', 'Y'
  )) %>%
  st_set_crs(st_crs(HI))

# connecting HI mask you made to your rec_box
connected_to_rec_box <- nngeo::st_connect(st_zm(connected_HI), rec_box)


ggplot() +
  geom_sf(data = connected_HI, fill = 'turquoise', alpha = .4) +
  geom_sf(data = connected_to_rec_box, color = 'red')

^{由reprex package (v0.3.0) 于 2021-04-04 创建}