leaflet

Introduction

Leaflet is one of the most popular Javascript libraries for creating interactive maps. The leaflet R package allows you to create your own leaflet maps without needing to know any Javascript!

Installation

# install.packages("leaflet")
library(leaflet)

This is my first Map

Getting started with leaflet is easy. The leaflet() function creates a map widget that you can store in a variable so that you can modify the map later on. You can add features to the map using the pipe operator (%>%) just like in dplyr. The addTiles() function adds mapping data from Open Street Map.

# !formatR

m <- leaflet() %>% addTiles()
m  # a map with the default OSM tile layer

Now I am pinning Oodi Helsinki Central Library in Finland

m <- m %>%
  addMarkers(lat=60.17391336310122, lng=24.938040247094637, 
             popup="OODI Library")
m

Making Custom Markers

The blue markers that leaflet comes packaged with may not be enough depending on what you’re mapping. Thankfully you can make your own markers from .png files.

oodiicon <- makeIcon(
  iconUrl = "https://www.cnba.it/contenuti/uploads/2019/09/Oodi-480x250.jpg",
  iconWidth = 31*215/230, iconHeight = 31,
  iconAnchorX = 31*215/230/2, iconAnchorY = 16
)
oodilocation <- data.frame(
  lat = c(60.17391336310122),
  lng = c(24.938040247094637))
oodilocation %>% 
  leaflet() %>%
  addTiles() %>%
  addMarkers(icon = oodiicon)

Making Custom Markers

The blue markers that leaflet comes packaged with may not be enough depending on what you’re mapping. Thankfully you can make your own markers from .png or ´.jpg´ files. Here you can see the libraries in the city center of Helsinki

iconlib <- makeIcon(
  iconUrl = "https://galeri7.uludagsozluk.com/215/kitap_295314.jpg",
  iconWidth = 31*215/230, iconHeight = 31,
  iconAnchorX = 31*215/230/2, iconAnchorY = 16
)
libraries <- data.frame(
  lat = c(60.17391336310122, 60.17087587765499, 60.1729233860828),
  lng = c(24.938040247094637, 24.950524515551646, 24.950313608918563))
libraries %>% 
  leaflet() %>%
  addTiles() %>%
  addMarkers(icon = iconlib)

Adding Multiple Popups

When adding multiple markers to a map, you may want to add popups for each marker. You can specify a string of plain text for each popup, or you can provide HTML which will be rendered inside of each popup.

librarysites <- c(
  "<a href='http://www.oodihelsinki.fi/'> Oodi Library </a>",
  "<a href='https://www.kansalliskirjasto.fi/'>The National Library of Finland</a>",
  "<a href='http://www.helsinki.fi/'>Helsinki University</a>"

)
libraries %>% 
  leaflet() %>%
  addTiles() %>%
  addMarkers(icon = iconlib, popup = librarysites)

Mapping Clusters

Sometimes you might have so many points on a map that it doesn’t make sense to plot every marker. In these situations leaflet allows you to plot clusters of markers using addMarkers(clusterOptions = markerClusterOptions()). When you zoom in to each cluster, the clusters will separate until you can see the individual markers.

df <- data.frame(lat = runif(20, min = 60.17, max = 60.27),
                 lng = runif(20, min = 24.94, max = 25.10))
df %>% 
  leaflet() %>%
  addTiles() %>%
  addMarkers(clusterOptions = markerClusterOptions())

Mapping Circle Markers

Instead of adding markers or clusters you can easily add circle markers using addCircleMarkers().

df <- data.frame(lat = runif(20, min = 60.17, max = 60.27),
                 lng = runif(20, min = 24.94, max = 25.10))
df %>% 
  leaflet() %>%
  addTiles() %>%
  addCircleMarkers()

Drawing Circles

You can draw arbitrary shapes on the maps you create, including circles and squares. The code below draws a map where the circle on each city is proportional to the population of that city.

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

Drawing Rectangles

You can add rectangles on leaflet maps as well: Here is the Kaisainiemenlahti

leaflet() %>%
  addTiles() %>%
  addRectangles(lat1 = 60.1733, lng1 = 24.9410, 
                lat2 = 60.1763, lng2 = 24.9450)

Adding Legends

Adding a legend can be useful if you have markers on your map with different colors:

df <- data.frame(lat = runif(20, min = 60.17, max = 60.27),
                 lng = runif(20, min = 24.94, max = 25.10),
                 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"))

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

Conclusion

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