[R Course] Data Visualization with R: Interactive Maps

R Courses Data Visualization

Learn to use the Leaflet R package that makes easy to integrate and control interactive maps.

Thierry Warin https://warin.ca/aboutme.html (HEC Montréal and CIRANO (Canada))https://www.hec.ca/en/profs/thierry.warin.html
08-28-2019

Basic Usage

You create a Leaflet map with these basic steps:

  1. Create a map widget by calling leaflet().
  2. Add layers (i.e., features) to the map by using layer functions (e.g. addTiles, addMarkers, addPolygons) to modify the map widget.
  3. Repeat step 2 as desired.
  4. Print the map widget to display it.

Without Marker

leaflet() %>% 
  setView(lng = -73.582189, lat = 45.517958, zoom = 13) %>% 
  addTiles()

With Marker

leaflet() %>% 
  addTiles() %>%
  addMarkers(lng=-73.582189, lat=45.517958, popup="The birthplace of SKEMA Quantum Studio")

Widget

The function leaflet() returns a Leaflet map widget, which stores a list of objects that can be modified or updated later. Most functions in this package have an argument map as their first argument, which makes it easy to use the pipe operator %>% in the magrittr package, as you have seen from the example in the Introduction.

Zoom

The map can be zoom in and out. You can play with the leafletOption as follow.

# Set value for the minZoom and maxZoom settings.
leaflet(options = leafletOptions(minZoom = 0, maxZoom = 18)) %>% 
  addTiles() %>%
  addMarkers(lng=-73.582189, lat=45.517958, popup="The birthplace of SKEMA Quantum Studio")

Data Object

Both leaflet() and the map layer functions have an optional data parameter that is designed to receive spatial data in one of several forms:

  1. From base R:
    • lng/lat matrix
    • data frame with lng/lat columns
  2. From the sp package: -SpatialPoints[DataFrame]
    • Line/Lines
    • SpatialLines[DataFrame]
    • Polygon/Polygons
    • SpatialPolygons[DataFrame]
  3. From the maps package:
    • the data frame from returned from map()

Here an example of the US map with colored states:

# Load the maps package (in addition to Leaflet package)
library(maps)
usa_states = map("state", fill = TRUE, plot = FALSE)
leaflet(data = usa_states) %>% 
  addTiles() %>%
  addPolygons(fillColor = topo.colors(10, alpha = NULL), stroke = FALSE)

Markers

Data sources

Point data for markers can come from a variety of sources:

Icon Markers

Icon markers are added using the addMarkers or the addAwesomeMarkers functions. Their default appearance is a dropped pin. As with most layer functions, the popup argument can be used to add a message to be displayed on click, and the label option can be used to display a text label either on hover or statically.

Leaflet allow you to add multiple markers on a map. As example, let’s create a data frame containing lngitude and latitude of SKEMA Business School. You have to search the lngitude and latitude for each wanted location.

# Create a data frame with lngitude and latitude of SKEMA Business School campuses. 
skema.campuses <- data.frame(country=c("United States", "Brazil", "South Africa", "China", "France", "France", "France"), 
                             city=c("Raleigh", "Belo Horizonte", "Cap Town", "Suzhou", "Paris", "Lille", "Sophia Antipolis"),
                             lng=c(-80.793457, -43.940933, 18.423300, 120.599998, 2.349014, 3.05858, 7.25),
                             lat=c(35.782169, -19.912998, -33.918861, 31.299999, 48.864716, 50.63297, 43.7))

skema.campuses
        country             city        lng       lat
1 United States          Raleigh -80.793457  35.78217
2        Brazil   Belo Horizonte -43.940933 -19.91300
3  South Africa         Cap Town  18.423300 -33.91886
4         China           Suzhou 120.599998  31.30000
5        France            Paris   2.349014  48.86472
6        France            Lille   3.058580  50.63297
7        France Sophia Antipolis   7.250000  43.70000
leaflet(data = skema.campuses) %>%
  addTiles() %>%
  addMarkers(lng=~lng, lat=~lat, popup=~country)

Customizing Marker Icons

You can provide custom markers in one of several ways, depending on the scenario. For each of these ways, the icon can be provided as either a URL or as a file path.

One color

For the simple case of applying a single icon to a set of markers, use makeIcon().

greenLeafIcon <- makeIcon(
  iconUrl = "http://leafletjs.com/examples/custom-icons/leaf-green.png",
  iconWidth = 38, iconHeight = 95,
  iconAnchorX = 22, iconAnchorY = 94,
  shadowUrl = "http://leafletjs.com/examples/custom-icons/leaf-shadow.png",
  shadowWidth = 50, shadowHeight = 64,
  shadowAnchorX = 4, shadowAnchorY = 62
)
leaflet(data = skema.campuses) %>%
  addTiles() %>%
  addMarkers(lng=~lng, lat=~lat, popup=~country, icon = greenLeafIcon)

Two colors

Let’s say that you want a different color for one (or more) location. For example, we want the french campus with a red leaf icon.

leafIcons <- icons(
  iconUrl = ifelse(skema.campuses$language == "French",
    "http://leafletjs.com/examples/custom-icons/leaf-red.png",
    "http://leafletjs.com/examples/custom-icons/leaf-green.png"
  ),
  iconWidth = 38, iconHeight = 95,
  iconAnchorX = 22, iconAnchorY = 94,
  shadowUrl = "http://leafletjs.com/examples/custom-icons/leaf-shadow.png",
  shadowWidth = 50, shadowHeight = 64,
  shadowAnchorX = 4, shadowAnchorY = 62
)

leaflet(data = skema.campuses) %>% addTiles() %>%
  addMarkers(~lng, ~lat, icon = leafIcons)

Multiple colors

# Create a column with the continents names for labeling in the map
skema.campuses$continent <- ifelse(skema.campuses$country == c("United States", "Brazil"), "Americas", ifelse(skema.campuses$country == "France", "Europe", "RoW"))



# Create a column containing a code (1, 2, 3 etc.) to match c
skema.campuses$code <- ifelse(skema.campuses$country == c("United States", "Brazil"), 1, 
                                ifelse(skema.campuses$country == "France", 2, 3))
getColor <- function(campuses) {
  sapply(skema.campuses$code, function(code) {
  if(code == 1) {
    "green"
  } else if(code == 2) {
    "red"
  } else {
    "orange"
  } })
}

icons <- awesomeIcons(
  icon = 'ios-close',
  iconColor = 'black',
  library = 'ion',
  markerColor = getColor(skema.campuses)
)

leaflet(skema.campuses) %>% addTiles() %>%
  addAwesomeMarkers(~lng, ~lat, icon=icons, label=~as.character(continent))

Marker Clusters

When there are a large number of markers on a map, you can cluster them using the Leaflet.markercluster plug-in. To enable this plug-in, you can provide a list of options to the argument clusterOptions, e.g.

SKEMA Business School has 3 campuses in France. Let’s add them on the map in a cluster.

leaflet(skema.campuses) %>% 
  addTiles() %>% 
  addMarkers(clusterOptions = markerClusterOptions())

When you zoom in, you can see the markers separately.

Circle Markers

Circle markers are much like regular circles, except that their radius in onscreen pixels stays constant regardless of zoom level.

You can use their default appearance:

leaflet(skema.campuses) %>% 
  addTiles() %>% 
  addCircleMarkers()

Or customize their color, radius, stroke, opacity, etc.

# Create a palette that maps factor levels to colors
pal <- colorFactor(c("navy", "red"), domain = c("English", "French"))

leaflet(skema.campuses) %>% 
  addTiles() %>%
  addCircleMarkers(
    radius = ~ifelse(language == "French", 6, 10),
    color = ~pal(language),
    stroke = FALSE, 
    fillOpacity = 0.5
  )

Popups

Popups are small boxes containing arbitrary HTML, that point to a specific point on the map.

You’ve already seen this example above:

leaflet(data = skema.campuses) %>%
  addTiles() %>%
  addMarkers(lng=~lng, lat=~lat, popup=~city)

Another way to have a popup is by using the addPopups() function to add standalone popup to the map.

content <- paste(sep = "<br/>",
  "<b><a href='https://quantumstudio.skemagloballab.io/'>SKEMA Quantum Studio</a></b>",
  "4200 Saint Laurent Blvd",
  "Montréal, QC H2W2R2"
)

leaflet() %>% addTiles() %>%
  addPopups(lng = -73.582189, lat = 45.517958, content,
    options = popupOptions(closeButton = FALSE)
  )

Labels

A label is a textual or HTML content that can attached to markers and shapes to be always displayed or displayed on mouse over. Unlike popups you don’t need to click a marker/polygon for the label to be shown.

library(htmltools)

leaflet(skema.campuses) %>% 
  addTiles() %>%
  addMarkers(~lng, ~lat, label = ~htmlEscape(country))

Customizing Marker Labels

You can customize marker labels using the labelOptions argument of the addMarkers function. The labelOptions argument can be populated using the labelOptions() function. If noHide is false (the default) then the label is displayed only when you hover the mouse over the marker; if noHide is set to true then the label is always displayed.

# Change Text Size and text Only and also a custom CSS
leaflet() %>% 
  addTiles() %>% 
  setView(-2.349014, 48.864716, 2) %>%
  addMarkers(
    lng = -80.793457, lat = 35.782169,
    label = "Default Label",
    labelOptions = labelOptions(noHide = T)) %>%
  addMarkers(
    lng = -43.940933, lat = -19.912998,
    label = "Label without surrounding box",
    labelOptions = labelOptions(noHide = T, textOnly = TRUE)) %>%
  addMarkers(
    lng = 18.423300, lat = -33.918861,
    label = "label with textsize 15px",
    labelOptions = labelOptions(noHide = T, textsize = "15px")) %>%
  addMarkers(
    lng = 120.599998, lat = 31.299999,
    label = "hidden label with textsize 12px",
    labelOptions = labelOptions(noHide = F, textsize = "10px")) %>%
  addMarkers(
    lng = 2.349014, lat = 48.864716,
    label = "Label w/ custom CSS style",
    labelOptions = labelOptions(noHide = T, direction = "bottom",
      style = list(
        "color" = "red",
        "font-family" = "serif",
        "font-style" = "italic",
        "box-shadow" = "3px 3px rgba(0,0,0,0.25)",
        "font-size" = "12px",
        "border-color" = "rgba(0,0,0,0.5)"
      )))

Labels without markers

You can create labels without the accompanying markers using the addLabelOnlyMarkers function.

leaflet() %>% 
  addTiles() %>%
  addLabelOnlyMarkers(data = skema.campuses, lng = ~lng, lat = ~lat, label = ~as.character(city), 
                      labelOptions = labelOptions(noHide = T, direction = 'top', textOnly = T))

Shapes

Circles

Circles are added using addCircles(). Circles are similar to circle markers; the only difference is that circles have their radii specified in meters, while circle markers are specified in pixels. As a result, circles are scaled with the map as the user zooms in and out, while circle markers remain a constant size on the screen regardless of zoom level.

When plotting circles, only the circle centers (and radii) are required, so the set of valid data sources is different than for polygons and the same as for markers. See the introduction to Markers for specifics.

# Adding (fake) student population 
skema.students <- data.frame(city=c("Raleigh", "Belo Horizonte", "Cap Town", "Suzhou", "Paris", "Lille", "Sophia Antipolis"),
                             students=c(800, 1500, 500, 2500, 1500, 1200, 1000))
library(dplyr)
skema.campuses <- left_join(skema.campuses, skema.students, by="city")
leaflet(skema.campuses) %>% 
  addTiles() %>%
  addCircles(lng = ~lng, lat = ~lat, weight = 10, radius = ~students * 200, popup = ~city)

Rectangles

Rectangles are added using the addRectangles() function. It takes lng1, lng2, lat1, and lat2 vector arguments that define the corners of the rectangles. These arguments are always required; the rectangle geometry cannot be inferred from the data object.

leaflet() %>% 
  addTiles() %>%
  addRectangles(
    lng1=-73.582189, lat1=45.517958,
    lng2=-73.582700, lat2=45.517500,
    fillColor = "transparent"
  )

Tiles

Leaflet supports basemaps using map tiles.

Default OpenStreetMap Tiles

The easiest way to add tiles is by calling addTiles() with no arguments; by default, OpenStreetMap tiles are used.

leaflet() %>% setView(lng = -73.656830 , lat = 45.516136, zoom = 12) %>% 
  addTiles()

Third-Party Tiles

Alternatively, many popular free third-party basemaps can be added using the addProviderTiles() function, which is implemented using the leaflet-providers plugin.

As a convenience, leaflet also provides a named list of all the third-party tile providers that are supported by the plugin. This enables you to use auto-completion feature of your favorite R IDE (like RStudio) and not have to remember or look up supported tile providers; just type providers$ and choose from one of the options. You can also use names(providers) to view all of the options.

leaflet() %>% setView(lng = -73.656830 , lat = 45.516136, zoom = 12) %>% 
  addProviderTiles(providers$Stamen.Toner)
leaflet() %>% setView(lng = -73.656830 , lat = 45.516136, zoom = 12) %>% 
  addProviderTiles(providers$CartoDB.Positron)
leaflet() %>% setView(lng = -73.656830 , lat = 45.516136, zoom = 12) %>% 
  addProviderTiles(providers$Esri.NatGeoWorldMap)

Providers Tiles

Leaflet provides a lot of different Tiles. You can found them here.

To use one of these tiles it’s simple. Add providers$ with the name of the desired tile in the funtion addProviderTiles().

Here some example:

OpenTopoMap

leaflet() %>% setView(lng = -73.656830 , lat = 45.516136, zoom = 9) %>% 
  addProviderTiles(providers$OpenTopoMap)

Stamen.Watercolor

leaflet() %>% setView(lng = -73.656830 , lat = 45.516136, zoom = 11) %>% 
  addProviderTiles(providers$Stamen.Watercolor)

NASAGIBS.ViirsEarthAtNight2012

leaflet() %>% setView(lng = -73.656830 , lat = 45.516136, zoom = 2.5) %>% 
  addProviderTiles(providers$NASAGIBS.ViirsEarthAtNight2012)

Esri.WorldTerrain

leaflet() %>% setView(lng = -73.656830 , lat = 45.516136, zoom = 3) %>% 
  addProviderTiles(providers$Esri.WorldTerrain)

Combining Tile Layers

You aren’t restricted to using a single basemap on a map; you can stack them by adding multiple tile layers. This generally only makes sense if the front tiles consist of semi transparent tiles, or have an adjusted opacity via the options argument.

leaflet() %>% setView(lng = -73.656830 , lat = 45.516136, zoom = 12) %>% 
  addProviderTiles(providers$MtbMap) %>%
  addProviderTiles(providers$Stamen.TonerLines, options = providerTileOptions(opacity = 0.35)) %>%
  addProviderTiles(providers$Stamen.TonerLabels)

Custom Tile URL Template

If you happen to have a custom map tile URL template to use, you can provide it as an argument to addTiles().

You can use addWMSTiles() to add WMS (Web Map Service) tiles. The map below shows the Base Reflectivity (a measure of the intensity of precipitation occurring) using the WMS from the Iowa Environmental Mesonet:

leaflet() %>% addTiles() %>% setView(-93.65, 42.0285, zoom = 4) %>%
  addWMSTiles(
    "http://mesonet.agron.iastate.edu/cgi-bin/wms/nexrad/n0r.cgi",
    layers = "nexrad-n0r-900913",
    options = WMSTileOptions(format = "image/png", transparent = TRUE),
    attribution = "Weather data © 2012 IEM Nexrad"
  )


Citation

For attribution, please cite this work as

Warin (2019, Aug. 28). www.warin.ca: [R Course] Data Visualization with R: Interactive Maps. Retrieved from https://warin.ca/posts/rcourse-datavisualizationwithr-interactivemaps/

BibTeX citation

@misc{warin2019[r,
  author = {Warin, Thierry},
  title = {www.warin.ca: [R Course] Data Visualization with R: Interactive Maps},
  url = {https://warin.ca/posts/rcourse-datavisualizationwithr-interactivemaps/},
  year = {2019}
}