Hey!

Today I would like to introduce you to the leaflet package for R. This package makes it easy to integrate and control Leaflet maps in R. In other words: You can create beautiful Leaflet maps with your data from inside R, without any knowledge of JavaScript, HTML, CSS, etc… Sounds amazing, right? This is how it works…
First let’s prepare some data:

library(raster)
#get boundaries
usa <- getData("GADM", country="USA", level=2)
#add random data
usa$randomData <- rnorm(n=nrow(usa), 150, 30)

The fastest way to get to some arbitrary data, is to load the raster package and download administrative boundaries of a country of your choice using the getData() function. Note that country boundaries come without any additional data (like population, income, etc..). Since we later want to visualise some data on our map using some colors and pop-ups, I added a new column to the SpatialPolygonsDataFrame and filled it with random data using the rnorm function.

Create the map

The easiest way to create an HTML page that contains a leaflet map is to use RMarkdown. For this purpose go to New File -> RMarkdown -> HTML. This is how the final RMarkdown document looks like:

---
title: "Leaflet with R 101"
output: html_document
---

```{r, echo=F, warning=F, message=F}
#load leaflet package for R
library(leaflet)
library(raster)

#arbitrary data (code from above)
usa <- getData("GADM", country="USA", level=2)
usa$randomData <- rnorm(n=nrow(usa), 150, 30)

#create a color palette to fill the polygons
pal <- colorQuantile("Greens", NULL, n = 5)

#create a pop up (onClick)
polygon_popup <- paste0("<strong>Name: </strong>", usa$NAME_1, "<br>",
                        "<strong>Indicator: </strong>", round(usa$randomData,2))

#create leaflet map
  map = leaflet() %>% 
      addProviderTiles("CartoDB.Positron") %>% 
      setView(-98.35, 39.7,
              zoom = 4) %>% 
      addPolygons(data = usa, 
                  fillColor= ~pal(randomData),
                  fillOpacity = 0.4, 
                  weight = 2, 
                  color = "white",
                  popup = polygon_popup)
map
```

As you can see it the code is short and quite intuitive:

leaflet() initialises a new leaflet map, tiles/basemape can be added using the addTiles() or addProviderTiles() functions. In my example I included a basemap provided by CartoDB called “Positron”. Once you set the view and the zoom level using the setView() function, you can overlay your data with the addPolygons() function. And that’s it! Click on KnitHTML and view the final result in your browser:

I hope you enjoyed this rather short introduction into the very powerful leaflet package for R. If you are intrested in more detailed tutorials or code examples, please leave a comment below or visit this official Leaflet for R – Introduction.

Cheers

Martin