在R中的坐标周围绘制多边形

Question

我有来自不同地理区域的不同数据集的点集群。我想在地图上绘制每个区域的空间范围，为此我需要每个区域一个多边形。我可以通过“连接点”找到将点转换为多边形的命令，但没有通过绘制轮廓来找到命令（几何方法并不是非常重要，因为这只是用于绘图）。这是一个生成简化数据框的表达式，就像我正在使用的那样，用于挪威、阿拉斯加和美国大陆的点：

nor_lat <- c(72.117, 71.05, 71.717, 71.167, 71.817, 72.333, 71.25, 70.917, 
  70.85, 70.933)
nor_lon <- c(33.217, 30.333, 26.3, 22.333, 22.333, 22.333, 18.517, 20.133, 
             21.333, 19.917)
us_lat <- c(41.283333, 41, 40.95, 40.95, 38.683333, 40.783333, 40.733333, 
            40.516667, 38.566667, 41.266667)
us_lon <- c(-71.116667, -70.733333, -70.583333, -70.483333, -74.816667, 
            -70.5, -70.566667, -70.266667, -74.85, -71.366667)
ak_lat <- c(58.015, 57.67167, 57.33833, 56.685, 56.99333, 57.31667, 57.65, 
            57.99667, 58.32167, 58.33333)
ak_lon <- c(-158.31667, -158.36, -158.41, -159.76, -159.72333, -159.66333, 
            -159.64167, -159.605, -159.54, -160.72167)
nor_dat <- data.frame(lat=nor_lat, lon=nor_lon)
us_dat <- data.frame(lat=us_lat, lon=us_lon)
ak_dat <- data.frame(lat=ak_lat, lon=ak_lon)
nor_dat$region <- "nor"
us_dat$region <- "us"
ak_dat$region <- "ak"
dat <- rbind(nor_dat, us_dat, ak_dat)

对于对象dat，我想在每个region 中的点周围生成一个简单的多边形，如果可能的话使用sf。 （我不太担心如何迭代这些区域，但我什至不知道如何围绕一组点绘制多边形！）

Answer 1

library(sf)
library(dplyr)

hulls <- dat %>%
  st_as_sf(coords = c("lon", "lat")) %>%
  group_by(region) %>%
  summarize(geometry = st_union(geometry)) %>%
  st_convex_hull()
plot(hulls)

Answer 2

一种方法使用sf 和dplyr 进行“地理计算”和过滤，使用tmap 和tmaptools 进行可视化。

请找到以下表示。

Reprex

• 将dataframe“dat”转换为sf对象

library(sf)
library(dplyr)
library(tmap)
library(tmaptools)

sf_use_s2(TRUE)

dat_sf <- st_as_sf(dat, coords = c("lon", "lat"), crs = 4326)

• 为每个区域中的每组点计算一个平滑的包络多边形

conv_nor <- st_simplify(st_buffer(st_convex_hull(st_union(st_geometry(filter(dat_sf, region == "nor")))), dist = 15000), dTolerance = 5000)
conv_us <- st_simplify(st_buffer(st_convex_hull(st_union(st_geometry(filter(dat_sf, region == "us")))), dist = 15000), dTolerance = 5000)
conv_ak <- st_simplify(st_buffer(st_convex_hull(st_union(st_geometry(filter(dat_sf, region == "ak")))), dist = 15000), dTolerance = 5000)

• 可视化：概述

tm_shape(dat_sf)+
  tm_dots(size = 0.2, col = "black", shape = 21, alpha = 0.3)+
  tm_shape(conv_nor)+
  tm_borders(col = "gray", lwd = 2)+
  tm_shape(conv_us)+
  tm_borders(col = "blue", lwd = 2)+
  tm_shape(conv_ak)+
  tm_borders(col = "gray", lwd = 2)+
  tm_layout(frame = FALSE)

• 可视化：详细视图

tm_shape(filter(dat_sf, region == "nor"), bbox = bb(conv_nor, ext = 1))+
  tm_dots(size = 0.5, alpha = 0.5)+
  tm_shape(conv_nor)+
  tm_borders(col = "gray", lwd = 2)+
  tm_layout(frame = FALSE, title = "'nor' area", 
            title.size = 1.5, title.position = c(0.85, "bottom"))

tm_shape(filter(dat_sf, region == "us"), bbox = bb(conv_us, ext = 1))+
  tm_dots(size = 0.5, alpha = 0.5)+
  tm_shape(conv_us)+
  tm_borders(col = "gray", lwd = 2)+
  tm_layout(frame = FALSE, title = "'us' area", 
            title.size = 1.5, title.position = c(0., "top"))

tm_shape(filter(dat_sf, region == "ak"), bbox = bb(conv_ak, ext = 1))+
  tm_dots(size = 0.5, alpha = 0.5)+
  tm_shape(conv_ak)+
  tm_borders(col = "gray", lwd = 2)+
  tm_layout(frame = FALSE, title = "'ak' area", 
            title.size = 1.5, title.position = c(0.75, "top"))

• 带背景的详细视图

最后，如果你想要一些上下文背景，你可以使用tmap_mode("view") 调用leaflet - 显然你在海洋环境中工作;-)

tmap_mode("view")
#> tmap mode set to interactive viewing
tm_shape(filter(dat_sf, region == "nor"), bbox = bb(conv_nor, ext = 1))+
  tm_dots(size = 0.5, alpha = 0.5)+
  tm_shape(conv_nor)+
  tm_borders(col = "gray", lwd = 2)+
  tm_layout(frame = FALSE, title = "'nor' area", 
            title.size = 1.5, title.position = c(0.85, "bottom"))

tm_shape(filter(dat_sf, region == "us"), bbox = bb(conv_us, ext = 1))+
  tm_dots(size = 0.5, alpha = 0.5)+
  tm_shape(conv_us)+
  tm_borders(col = "gray", lwd = 2)+
  tm_layout(frame = FALSE, title = "'us' area", 
            title.size = 1.5, title.position = c(0., "top"))

tm_shape(filter(dat_sf, region == "ak"), bbox = bb(conv_ak, ext = 1))+
  tm_dots(size = 0.5, alpha = 0.5)+
  tm_shape(conv_ak)+
  tm_borders(col = "gray", lwd = 2)+
  tm_layout(frame = FALSE, title = "'ak' area", 
            title.size = 1.5, title.position = c(0.75, "top"))

tmap_mode("plot")
#> tmap mode set to plotting

^{由reprex package (v2.0.1) 于 2021-10-20 创建}