在 R 中围绕坐标绘制多边形

Question

I have clusters of points from different datasets collected in different geographical regions. I'd like to plot the spatial extent of each region on a map, for which I need a single polygon per region. I can find commands to convert points to polygons by "connecting the dots", but none by drawing an outline (the geometric method isn't terribly important since this is just for plotting). Here's a reprex to generate a simplified dataframe like the one I'm using, for points in Norway, Alaska, and the continental US:

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)

For the object dat , I'd like to generate a simple polygon around the points in each region , using sf if possible. (I'm not too worried about figuring out how to iterate over the regions, but I can't even figure out how to draw a polygon around one set of points!)

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

One approach using sf for "geocomputation", tmap and tmaptools for visualization.

Please find the following reprex.

Reprex

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

sf_use_s2(TRUE)

• Convert nor_dat , us_dat , ak_dat and dat into sf object

nor_sf <- st_as_sf(nor_dat, coords = c("lon", "lat"), crs = 4326)
us_sf <- st_as_sf(us_dat, coords = c("lon", "lat"), crs = 4326)
ak_sf <- st_as_sf(ak_dat, coords = c("lon", "lat"), crs = 4326)
dat_sf <- st_as_sf(dat, coords = c("lon", "lat"), crs = 4326)

• Compute a smoothed enveloping polygon for each set of points in each area

conv_nor <- st_simplify(st_buffer(st_convex_hull(st_union(st_geometry(nor_sf))), dist = 15000), dTolerance = 5000)
conv_us <- st_simplify(st_buffer(st_convex_hull(st_union(st_geometry(us_sf))), dist = 15000), dTolerance = 5000)
conv_ak <- st_simplify(st_buffer(st_convex_hull(st_union(st_geometry(ak_sf))), dist = 15000), dTolerance = 5000)

• Visualization: Overview

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)

• Visualization: detailed views

tm_shape(nor_sf, 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(us_sf, 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(ak_sf, 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"))

• And finally, if you want some contextual background, you can use the tmap_mode("view") which call leaflet - apparently you work on the marine environment ;-)

tmap_mode("view")
#> tmap mode set to interactive viewing
tm_shape(nor_sf, 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(us_sf, 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(ak_sf, 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

^{Created on 2021-10-20 by the reprex package (v2.0.1)}