Chapter 3 Leaflet

3.1 Getting Started

Leaflet is a Javascript library that allows users to create interactive maps that even look good on mobile devices. The wonderful folks at RStudio created their own Leaflet R package which allows R users to create their own Leaflet maps without needing to write any Javascript. Let’s get started making some maps!

First you should of course install Leaflet:

install.packages("leaflet")

Once you have leaflet installed you can start making maps! You can store your map in a variable like any other value, in this exmaple let’s store our map in a variable called my_map. You need to use the addTiles() function to add a base layer to your map, and you can use the pipe operator (%>%) to sequentially add features to your map. Here’s how you can make a very simple, basic map:

library(leaflet)

my_map <- leaflet() %>%
  addTiles()
my_map

Whoa, it’s the whole world! In the HTML version of this book you should be able to move the map around and zoom in and out as you please. You’ve made your first interactive map! Leaflet uses free and open source data from the OpenStreetMap project.

As lovely as the whole world is, you might want to specifiy a particular location and zoom level if you’re trying to show a particular location on your map. You can do this with the setView() function by specifying the longitude, latitude, and the zoom level:

my_map <- my_map %>%
  setView(lat = 39.2898036, lng = -76.6051842, zoom = 13)
my_map

Ah look it’s beautiful Baltimore, the Greatest City in America!

3.2 Adding Markers

This is certainly a more interesting map than we had before, but it’s still lacking features. Let’s add a marker to this map using the addMarkers() function:

my_map <- my_map %>%
  addMarkers(lat=39.2980803, lng=-76.5898801, 
             popup="Jeff Leek's Office")
my_map

As you can see you can add a marker just by specifying longitude and latitude. Optionally you can add some pop up text which appears when you click on the marker. You don’t have to add one marker at a time. To create multiple markers at once, use a data frame with columns named lat and lng for latitude and longitude:

suppressWarnings(set.seed(2016-04-25))
df <- data.frame(lat = runif(20, min = 39.2, max = 39.3),
                 lng = runif(20, min = -76.6, max = -76.5))
df %>% 
  leaflet() %>%
  addTiles() %>%
  addMarkers()

As you can see you can use the pipe operator to pipe a data frame directly into leaflet().

The default blue “teardrop” icon that Leaflet provides is nice, but you can also create your own icon with the makeIcon() function. You’ll need to provide a few arguments to this function including a link to the URL for your icon, plus width and height variables. Once you’ve made an icon object you can use that icon by specifying the icon argument in addmarkers():

hopkinsIcon <- makeIcon(
  iconUrl = "http://brand.jhu.edu/content/uploads/2014/06/university.shield.small_.blue_.png",
  iconWidth = 31*215/230, iconHeight = 31,
  iconAnchorX = 31*215/230/2, iconAnchorY = 16
)

hopkinsLatLong <- data.frame(
  lat = c(39.2973166, 39.3288851, 39.2906617, 39.2970681, 39.2824806),
  lng = c(-76.5929798, -76.6206598, -76.5469683, -76.6150537, -76.6016766))

hopkinsLatLong %>% 
  leaflet() %>%
  addTiles() %>%
  addMarkers(icon = hopkinsIcon)

Now we’ve nicely mapped out all of the Johns Hopkins campus locations around the city of Baltimore. You can customize text pop ups by including any valid HTML code in a vector. Then you can provide that vector as the popup argument in addMarkers() like so:

hopkinsSites <- c(
  "<a href='http://www.jhsph.edu/'>East Baltimore Campus</a>",
  "<a href='https://apply.jhu.edu/visit/homewood/'>Homewood Campus</a>",
  "<a href='http://www.hopkinsmedicine.org/johns_hopkins_bayview/'>Bayview Medical Center</a>",
  "<a href='http://www.peabody.jhu.edu/'>Peabody Institute</a>",
  "<a href='http://carey.jhu.edu/'>Carey Business School</a>"
)

hopkinsLatLong %>% 
  leaflet() %>%
  addTiles() %>%
  addMarkers(icon = hopkinsIcon, popup = hopkinsSites)

Each Johns Hopkins campus location now has a pop up with a link to the website for its respective campus!

df <- data.frame(lat = runif(500, min = 39.25, max = 39.35),
                 lng = runif(500, min = -76.65, max = -76.55))
df %>% 
  leaflet() %>%
  addTiles() %>%
  addMarkers(clusterOptions = markerClusterOptions())

df <- data.frame(lat = runif(20, min = 39.25, max = 39.35),
                 lng = runif(20, min = -76.65, max = -76.55))
df %>% 
  leaflet() %>%
  addTiles() %>%
  addCircleMarkers()

3.3 Drawing

md_cities <- data.frame(name = c("Baltimore", "Frederick", "Rockville", "Gaithersburg", 
                                 "Bowie", "Hagerstown", "Annapolis", "College Park", "Salisbury", "Laurel"),
                        pop = c(619493, 66169, 62334, 61045, 55232,
                                39890, 38880, 30587, 30484, 25346),
                        lat = c(39.2920592, 39.4143921, 39.0840, 39.1434, 39.0068, 39.6418, 38.9784, 38.9897, 38.3607, 39.0993),
                        lng = c(-76.6077852, -77.4204875, -77.1528, -77.2014, -76.7791, -77.7200, -76.4922, -76.9378, -75.5994, -76.8483))
md_cities %>%
  leaflet() %>%
  addTiles() %>%
  addCircles(weight = 1, radius = sqrt(md_cities$pop) * 30)

## Assuming 'lng' and 'lat' are longitude and latitude, respectively

leaflet() %>%
  addTiles() %>%
  addRectangles(lat1 = 37.3858, lng1 = -122.0595, 
                lat2 = 37.3890, lng2 = -122.0625)

df <- data.frame(lat = runif(20, min = 39.25, max = 39.35),
                 lng = runif(20, min = -76.65, max = -76.55),
                 col = sample(c("red", "blue", "green"), 20, replace = TRUE),
                 stringsAsFactors = FALSE)

df %>%
  leaflet() %>%
  addTiles() %>%
  addCircleMarkers(color = df$col) %>%
  addLegend(labels = LETTERS[1:3], colors = c("blue", "red", "green"))

3.4 Conclusion

For more details about the leaflet package for R visit http://rstudio.github.io/leaflet/.