在预定义的罗宾逊地图上绘制国家之间的线

Question

我有一张特定的地图，我想在其中添加国家之间的线条，然后再添加线条粗细。下面的代码运行，但地图上没有显示任何内容。请注意，

df

是用

ne_countries()

创建的，但我

st_transform

将地图映射到罗宾逊投影，所以我不确定df的坐标是否正确。

我知道在地图上绘制线条的方法有很多，但我不确定 geom_segment 是最好的，所以我愿意接受其他解决方案。我可以先添加点，然后再添加线，我已经见过这种方法，但也无法实现。

library(ggplot2)
library(sf)
library(rnaturalearth)
library(dplyr)
library(rgdal)

#creating the map:

# Download countries polygons
shp <- ne_countries(type = "countries", 
                    scale = "medium",
                    returnclass = "sf") |>
  # transform to desired projection
  st_transform("ESRI:54030")

shp <- subset(shp, select = -c(1:17, 19:63))

# Creating df with long and lat

countries <- ne_countries()

countries$longitude <- coordinates(countries)[,1]

countries$latitude <- coordinates(countries)[,2]

countries_xy <- countries@data %>%
  select(admin, longitude, latitude)


# Joining long and lat to origin & destination

df1 <- tibble(origins = sample(c('Australia'), 
                                  size = 1, replace = TRUE), 
                 destinations = sample(c('Fiji', 'Papua New Guinea', 'Sweden'), 
                                       size = 3, replace = FALSE))

df2 <- df1 %>%
  left_join(countries_xy, by = c('origins' = 'admin')) %>%
  left_join(countries_xy, by = c('destinations' = 'admin'))

df2$longitude.y <- as.numeric(as.character(df3$longitude.y))

df2$latitude.y <- as.numeric(as.character(df3$latitude.y))

# Merging shp and df

shp <- merge(shp,df2, by = "name", all.x = TRUE)

# Drawing the lines on the map (note that I also color the map with a discrete variable, but I don't think that is important here)

shp |>
  ggplot() +
  geom_sf(aes(fill = discrete_var)) +
  geom_segment(aes(x = latitude.x, 
                   y = latitude.y,
                   xend = longitude.x,
                   yend = longitude.y), 
               color = "white") +
  scale_fill_brewer(palette = 'Purples', direction=-1, na.value="grey", drop = FALSE) +
  coord_sf(expand = FALSE)
  

我知道还有很多其他在地图上画线的方法，但我不确定

geom_segment

是最好的，所以我愿意接受其他解决方案。

Answer 1

如果空间数据都是空间对象（sf、sfc、SpatRaster 等），则绘制和分析空间数据会更容易。因此，这里有一种将线条创建为 sf 对象的方法。

从您的代码中不清楚您希望行在哪里终止，并且您的代码中存在一些语法错误，因此我在此示例中任意使用了国家/地区质心。如果您愿意，可以使用

st_nearest_points()

，但我相信质心对于您要映射的内容来说是视觉上更好的选择。如果您希望线路在不同位置终止，请在下面评论。

library(sf)
library(dplyr)
library(rnaturalearth)
library(ggplot2)

# Create world map in Robinson projection
countries <- ne_countries() %>%
  st_transform("ESRI:54030")

# Subset countries of interest and add lines between Australia and the rest
sf_line <- countries %>%
  select(admin) %>%
  filter(admin %in% c("Australia", "Fiji", "Papua New Guinea", "Sweden")) %>%
  mutate(to = st_centroid(geometry)) %>%
  st_set_geometry("to") %>%
  select(-geometry) %>%
  mutate(from = to[admin == "Australia"],
         line_geom = st_union(to, from, by_feature = TRUE)) %>%
  filter(!admin == "Australia") %>%
  st_set_geometry("line_geom") %>%
  st_cast("LINESTRING")

ggplot() +
  geom_sf(data = countries, colour = "grey50") +
  geom_sf(data = sf_line, colour = "#CD0BBC", linewidth = 1)