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Using custom tiles in an RStudio Leaflet map

· by Kyle Walker · Read in about 16 min · (3310 Words)

Yesterday, I posted to Twitter an interactive map using the classic John Snow Cholera dataset and tiles made from Snow’s map, which attracted a fair share of interest.

I was inspired to try this by Lincoln Mullen’s tweet that custom historical tiles could be used in an RStudio Leaflet map. Incidentally, I am teaching students in my Advanced GIS course at TCU about tiled mapping next week; I was (and still am) going to use MapBox Studio as an example, but I thought this might work as a compelling example as well, especially as I assigned my students a lab assignment earlier in the semester in which they made kernel density maps from the Snow data.

Below, I’ll explain how I got this done. My data come from the Yale University Library’s GIS Workshop Archive, in which they generously make the John Snow data publicly available. However, the data are stored in an Esri File Geodatabase, for which there does not appear to be an open-source solution to get rasters out of yet; as such, I’ve placed the John Snow map image as a GeoTIFF and the deaths point data as a shapefile in this GitHub repository. The Snow map is already georectified, which means that it has been georeferenced to a coordinate system and exported as a geographic dataset. I’m not going to go over georeferencing here if you are working with a plain image; you can read more about how to do that in QGIS here.

To get started, I needed to create tiles from my image. Modern web maps in the style of Google Maps employ tiled mapping to render basemaps quickly; you can read more about tiled mapping here. In a nutshell, tiled maps are tessellations of 256px by 256px images that are referenced based on their zoom level and coordinates.

To create tiles from my John Snow map image, I used the gdal2tiles utility. gdal2tiles is a command-line tool available within the Geospatial Data Abstraction Library, which is a suite of command-line functions for manipulating geographic data. Fortunately, if you have QGIS on your machine, you have GDAL installed. The easiest way to access it is to open the OSGeo4W Shell where your QGIS installation is located, which will give you access to the tools.

In the OSGeo4W shell, I cd into my directory where my John Snow image is located and enter the following command:

gdal2tiles -s EPSG:32630 -z 14-18 snow.tif snow

The -s option refers to the input coordinate system of the georectified image; mine is in WGS84 UTM Zone 30N, which has the EPSG code of 32630. The -z option governs the zoom levels for which tiles will be generated; as my map covers a relatively small area, I’m sticking with zoom levels 14 to 18. I then specify an input image file (‘snow.tif’) and an output directory that I am calling snow which the function will create for me.

After the tool is finished running, I have a directory, snow, that is populated with PNG images, organized by their zoom level and coordinates. My snow directory looks like this:

+---14
|   +---8185
|   \---8186
+---15
|   +---16370
|   +---16371
|   \---16372
+---16
|   +---32741
|   +---32742
|   +---32743
|   \---32744
+---17
|   +---65483
|   +---65484
|   +---65485
|   +---65486
|   +---65487
|   \---65488
\---18
    +---130966
    +---130967
    +---130968
    +---130969
    +---130970
    +---130971
    +---130972
    +---130973
    +---130974
    +---130975
    +---130976
    \---130977

I have a folder for each zoom level, organized into sub-folders that are designated by their tile X coordinates. Within each of these subfolders are PNG images, the tiles themselves, that are named by their tile Y coordinates. Here’s an example of a tile, “174980.png” at 18/130970/174980:

tile image

tile image

Now, I need a place to host my tiles so they can be consumed by a web mapping API like Leaflet. To do this, I’ve created a GitHub repository and published the tiles using GitHub Pages, which is what I use to host my data visualization blog.

I’m now ready to create my map in RStudio! I’m just creating a basic map with the Snow map as a basemap, circles sized proportionately to the number of cholera deaths at a given location, and a pop-up on click that shows the number of deaths. The code below gets this done:

library(leaflet)
library(rgdal)

dir <- getwd()

deaths <- readOGR(dir, "deaths", verbose = FALSE)

deathsxy <- spTransform(deaths, CRS("+proj=longlat +datum=WGS84"))

circle_popup <- paste0("<strong>Address: </strong>", 
                      deathsxy$Address, 
                      "<br><strong>Number of deaths: </strong>", 
                      deathsxy$Num_Cases)

leaflet() %>%
  setView(-0.1354223, 51.5135085, zoom = 17) %>%
  addTiles(urlTemplate = "http://walkerke.github.io/tiles/snow/{z}/{x}/{y}.png",
           attribution = 'Data source: <a href="http://guides.library.yale.edu/gisworkshoparchive">Yale University Library</a>', 
           options = tileOptions(minZoom = 15, maxZoom = 18, tms = TRUE)) %>%
  addCircles(data = deathsxy, 
             radius = deathsxy$Num_Cases, 
             popup = circle_popup, 
             color = "red", 
             fillColor = "red")

I won’t go into detail here about the syntax for the spatial data; check my other RPubs for tutorials on how to get spatial data into Leaflet. I will, however, explain how I incorporate my custom tiles. Note the addTiles function with the urlTemplate parameter. This allows you to use tiles other than the default OpenStreetMap tileset. The key is, then, to specify how the tiles will be retrieved. I use the following URL:

http://walkerke.github.io/tiles/snow/{z}/{x}/{y}.png

Notice how z, x, and y are enclosed within curly braces; this allows these values to vary as the user pans and zooms around the map. Depending on the zoom level and extent of the map, Leaflet will retrieve appropriate tiles; notice how this corresponds to the names of my directories and images in my snow folder. Incorporating other external tilesets in your maps works similarly; see this GitHub Gist for the syntax for some popular ones. Of course, you should always cite your sources afterward, which can be accomplished with the attribution parameter. Finally, if you have generated your tiles with gdal2tiles, be sure to set the tms parameter to TRUE with tileOptions, or else your tiles won’t show up.

If you have any comments or questions, please let me know!