How to batch printouts of a ggmap at maximum zoom level within a defined area

Aim: Print several maps at the maximum zoom layer from ggmap within a specific area.

Consider the white-shaded area.

It was made by adding a sf polygon to a ggmap (zoom level=14).

We would like to manually count (for lack of a better system), certain tree types in certain places. We can somewhat do that using zoom level 21.

Question:

How can we take the larger polygon, subset it into as many sections with zoom level 21 (maximum) and print them all out automatically.

Here is the polygon in question:

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41.5317566599916, 41.5316425105264), .Dim = c(314L, 2L))), class = c("XY", 
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"proj4string"), class = "crs"), class = c("sfc_POLYGON", "sfc"
), bbox = structure(c(2.18797068613092, 41.5239241289351, 2.23369093739271, 
41.5759472867263), .Names = c("xmin", "ymin", "xmax", "ymax"), class = "bbox"))

Here is the ggmap:

#Coordinates for Mollet del Vallès
mdv<-c(lon=2.2089,lat=41.5451)
#Download the relevant map
png(filename = "big_map.png",width=1280,height=1280)
big_map<-get_map(location=mdv,zoom=14,maptype="satellite",source="google")

gen_mdv<-ggmap(big_map) + 
geom_sf(data = mdv_pol, inherit.aes = FALSE,alpha=0.3)

This is a step to solve an urban forestry problem so maybe @aspiringurbandatascientist might have an idea.

Solution

This should do the trick:

Get required cell size

small_map <- get_map(location = mdv, zoom = 21, maptype = "satellite", source = "google")
bb <- attr(small_map, 'bb')
cellsize <- c(diff(bb[c(1, 3)] %>% unlist), diff(bb[c(2,4)] %>% unlist))

Get grid using st_make_grid (centre points) and intersect with polygon

poly.grid <- st_make_grid(polygon, what = 'centers', cellsize = cellsize) %>%
  st_intersection(polygon) 

ggplot() + 
  geom_sf(data = polygon %>% st_sf) + 
  geom_sf(data = poly.grid %>% st_sf, size = 0.1)

Get vector coordinates for each point as element of list

poly.grid <- lapply(poly.grid, function(x) st_coordinates(x))

Generate list of small maps at zoom 21 for each grid centre point. This will take a while.

small_maps <- lapply(poly.grid, function(x) get_map(location = x, zoom = 21, maptype = "satellite", source = "google")) 

ggmap(small_maps[[1]])

to print, use ggsave. You can adjust filename and other arguments passed to ggsave like resolution, width, height:

lapply(seq_along(small_maps), function(x) {
  ggsave(filename = paste0("map_", x,".png"), plot = ggmap(small_maps[[x]]), device = 'png')
})

make sure to read the help of ggsave, e.g. ?ggsave. Width and height arguments are in inches so don't use width = 1000!

Another tip: use lapply(seq_along(small_maps[1:2]), ...) to only try it on a subset of the maps. Where ... includes the rest of the function (I got lazy) :)