如何从leaflet或tmap中裁剪指定区域（以太浩湖为例）

要从大地图中提取太浩湖区域，关键是先获取太浩湖的地理边界，再基于边界限制地图显示范围。以下是适配你现有代码的具体实现方案：

1. 准备太浩湖边界数据 #

首先通过osmdata包从OpenStreetMap获取太浩湖的矢量边界（若未安装该包先执行安装命令）：

install.packages(c("osmdata", "dplyr"))
library(osmdata)
library(dplyr)
library(sf)

# 获取太浩湖的多边形边界
tahoe_bbox <- getbb("Lake Tahoe")
tahoe_poly <- opq(bbox = tahoe_bbox) %>%
  add_osm_feature(key = "natural", value = "water") %>%
  osmdata_sf() %>%
  .$osm_polygons %>%
  filter(name == "Lake Tahoe")

2. 修改tmap代码实现裁剪 #

基于你现有的tmap代码，添加太浩湖边界并锁定视图范围，就能只显示目标区域：

library(tmap)
library(tmaptools)

test <- tm_basemap("Esri.DeLorme") + 
  tm_shape(tahoe_poly) + tm_fill("blue", alpha = 0.3) +  # 叠加太浩湖区域
  tm_view(set.view = st_bbox(tahoe_poly))  # 锁定视图到太浩湖范围
test

# 导出裁剪后的地图图片
tmap_save(test, filename = "tahoe_tmap.png", width = 8, height = 6)

3. 修改leaflet代码实现裁剪 #

针对你提供的leaflet代码，通过fitBounds方法限制地图只能在太浩湖范围内显示：

library(leaflet)

# 使用Esri.DeLorme底图的裁剪版本
leaflet(tahoe_poly) %>%
  addProviderTiles("Esri.DeLorme") %>%
  addPolygons(fillColor = "blue", fillOpacity = 0.3, color = "black", weight = 2) %>%
  fitBounds(
    lng1 = st_bbox(tahoe_poly)[["xmin"]],
    lat1 = st_bbox(tahoe_poly)[["ymin"]],
    lng2 = st_bbox(tahoe_poly)[["xmax"]],
    lat2 = st_bbox(tahoe_poly)[["ymax"]]
  )

# 或者使用默认底图的裁剪版本
leaflet(tahoe_poly) %>% 
  addTiles() %>%  
  addPolygons(fillColor = "blue", fillOpacity = 0.3, color = "black", weight = 2) %>%
  fitBounds(
    lng1 = st_bbox(tahoe_poly)[["xmin"]],
    lat1 = st_bbox(tahoe_poly)[["ymin"]],
    lng2 = st_bbox(tahoe_poly)[["xmax"]],
    lat2 = st_bbox(tahoe_poly)[["ymax"]]
  )

如果需要导出leaflet的裁剪区域图片，可以用mapview包的mapshot工具：

install.packages("mapview")
library(mapview)

tahoe_map <- leaflet(tahoe_poly) %>%
  addProviderTiles("Esri.DeLorme") %>%
  addPolygons(fillColor = "blue", fillOpacity = 0.3) %>%
  fitBounds(st_bbox(tahoe_poly)[1], st_bbox(tahoe_poly)[2], st_bbox(tahoe_poly)[3], st_bbox(tahoe_poly)[4])

mapshot(tahoe_map, file = "tahoe_leaflet.png", cliprect = "viewport")

内容的提问来源于stack exchange，提问作者Salvador