Cleaned data on fatal automobile accident data for 2001–2010 here (click on the “Download” button on the page).

library(tigris)
denver_tracts <- tracts(state = "CO", county = 31, 
                        cb = TRUE, class = "sf")

load("../data/fars_colorado.RData") # Use relative path on your computer 
library(tidyverse)
accident_data <- driver_data %>%
  select(state, st_case, county, latitude, longitud,
                date, fatals, drunk_dr) %>%
  filter(county == 31 & longitud < -104.1) %>%
  distinct()

“Leaflet” is a JavaScript library for making interactive maps. You can find out more about the JavaScript version here: http://leafletjs.com

The leaflet package brings this functionality to R. The R Studio group has created a website on leaflet: http://rstudio.github.io/leaflet/. This website walks you through different options available with leaflet.

library(leaflet)

If you just run leaflet(), you just get a blank leaflet area:

leaflet()

In leaflet, the map background is composed of tiles. To get something more interesting, you’ll need to add tiles to your leaflet map. If you don’t include any other data, the leaflet map will include the world:

leaflet() %>%
  addTiles()

For htmlWidgets, points are often referred to as markers.

Once you add these markers, the map will automatically scale to a reasonable size for their bounding box.

leaflet() %>%
  addTiles() %>%
  addMarkers(data = accident_data, lng = ~ longitud, lat = ~ latitude)

Use lng and lat to tell R which columns contain data on longitude and latitude for each point. This is not needed if you are using a spatial object (e.g., SpatialPointsDataFrame). Further, R will try to guess the columns in a regular dataframe.

You can use several types of R objects for your data for leaflet:

• Dataframe with columns for latitude and longitude
• Simple feature objects
• Latitude-longitude matrix

You can choose circles for your markers instead by using addCircleMarkers. You can adjust the circle size with radius.

leaflet() %>%
  addTiles() %>%
  addCircleMarkers(data = accident_data, radius = 2,
                   lng = ~ longitud, lat = ~ latitude)

The radius argument specifies the size of the circle. For CircleMarkers, the size will reset as you zoom in and out. If you want something with a constant radius (e.g., in meters), you can add Circles.

If you have a lot of overlapping data, you can also use the clusterOptions argument to show the markers as clusters that group together when you zoom out but split up when you zoom in:

leaflet() %>%
  addTiles() %>%
  addMarkers(data = accident_data, 
                   lng = ~ longitud, lat = ~ latitude,
                   clusterOptions = markerClusterOptions())

For the background, the default is to use map tiles from OpenStreetMap. However, you can change the source of the tiles by using addProviderTiles. For example, to use Stamen Watercolor, you can call:

leaflet() %>%
  addProviderTiles("Stamen.Watercolor") %>%
  addCircleMarkers(data = accident_data, radius = 2,
                   lng = ~ longitud, lat = ~ latitude)

leaflet() %>%
  addProviderTiles("Esri.WorldStreetMap") %>%
  addCircleMarkers(data = accident_data, radius = 2,
                   lng = ~ longitud, lat = ~ latitude)

You can see previews of provider choices here: http://leaflet-extras.github.io/leaflet-providers/preview/index.html.