无法在R的地理地图上覆盖郡县？

Question

我正在尝试使用in an earlier post中提供的代码来生成明尼苏达州及其县的地图。

虽然我可以使用以下代码生成明尼苏达州的地图：

Minnesota<-get_map(location = "Minnesota", 
               zoom = 6, source = "google", maptype="roadmap")

我无法使用以下代码生成县的细分：

##Get Minnesota Counties
    counties <- map_data("county")
    mn_county <- subset(counties, region == 'minnesota')
    Minnesota + 
      geom_polygon(data = mn_county, aes(x=long, y=lat, group = group), fill = NA, color = "red")

而不是得到带有县轮廓的地图，而是得到以下错误消息：

Error in Minnesota + geom_polygon(data = mn_county, aes(x = long, y = lat,  : 
  non-numeric argument to binary operator
In addition: Warning message:
Incompatible methods ("Ops.raster", "+.gg") for "+" 

Answer 1

您必须使用ggmap(Minnesota)来映射base_layer：

library(ggplot2)
library(ggmap)

Minnesota<-get_map(location = "Minnesota", 
                   zoom = 6, source = "google", maptype="roadmap")

##Get Minnesota Counties
counties <- map_data("county")
mn_county <- subset(counties, region == 'minnesota')
ggmap(Minnesota) + 
  geom_polygon(data = mn_county, aes(x=long, y=lat, group = group), fill = NA, color = "red")

^{由reprex package（v0.3.0）在2020-03-28创建}