get_googlemap: I need to calculate the bounds of a map given center and zoom level

Question

I get the map using get_googlemap

location <- c(lon = -87.67946, lat = 41.86242)
map <- ggmap(get_googlemap(center = location, zoom = 11), extent = "device")

Once I have a ggmap, I want to learn what are the exact boundaries of my map to further use it for creating a heatmap from data with coordinates.

Please, help me calculate the bounds of a map given center and zoom level.

I tried looking for answer in ?get_googlemap and ?ggmap documentations but so far no luck.

Answer 1

The data member of the object map contains lat and lon columns that specify the bounds:

library(ggmap)

location <- c(lon = -87.67946, lat = 41.86242)
map <- ggmap(get_googlemap(center = location, zoom = 11), extent = "device")

range(map$data$lon)
#> [1] -87.89884 -87.45939

range(map$data$lat)
#> [1] 41.69831 42.02560

To demonstrate this is right, let's plot a red rectangle around our map using these numbers:

map +
  annotate("rect", xmin = min(map$data$lon), xmax = max(map$data$lon),
           ymin = min(map$data$lat), ymax = max(map$data$lat), fill = NA,
           color = "red", linewidth = 3)

Or if you want to create a grid over the map,

map + 
  geom_hline(yintercept = seq(min(map$data$lat), max(map$data$lat), len = 10)) +
  geom_vline(xintercept = seq(min(map$data$lon), max(map$data$lon), len = 10)) 

^{Created on 2023-07-15 with reprex v2.0.2}