Slippy Map Tile Composite

tile-composite.R

library(slippymath)
library(sf)
library(purrr)
library(curl)
library(glue)
library(raster)
library(rgdal)

# pass map.bounds() & zoom from leaflet
# xmin <- map.getBounds()["_southWest"]['lat']
# xmax <- map.getBounds()["_northEast"]['lat']
# ymin <- map.getBounds()["_southWest"]['lng']
# ymax <- map.getBounds()["_northEast"]['lng']

# get a bbox
outlook_bbox <-
  st_bbox(c(xmin = -122.35688209533693,
            xmax = -122.32357978820802,
            ymin =  47.64424145665395,
            ymax =  47.663898895823536),
          crs = sp::CRS("+init=epsg:4326"))

# slippy math to tile grid
max_tiles_param <- 15 # set programatically?
bbox_to_tile_grid(outlook_bbox, max_tiles = max_tiles_param)
tile_grid <- bbox_to_tile_grid(outlook_bbox, max_tiles = max_tiles_param)

# esri free tile server (used in leaflet)
# can be configurable to any of these: http://leaflet-extras.github.io/leaflet-providers/preview/
esri_query_string <-
  paste0("https://server.arcgisonline.com/ArcGIS/rest/services/World_Topo_Map/MapServer/tile/{zoom}/{y}/{x}")

# create tmp_dir
dir.create('tmp_tiles/')

# curl images
images <-
  pmap(tile_grid$tiles,
       function(x, y, zoom){
         outfile <- glue("tmp_tiles/{x}_{y}.jpg")
         curl_download(url = glue(esri_query_string),
                       destfile = outfile) 
         outfile 
       },
       zoom = tile_grid$zoom)

# returns a raster brick
raster_out <- compose_tile_grid(tile_grid, images)

# remove the curled tmp_tiles
unlink('tmp_tiles', recursive=TRUE)

# make a png to preview
raster_to_png(raster_out, "curr_leaflet_view.png")