Leaflet for Interactive Maps

Today we will create interactive maps.

OpenStreetMap (OSM) is a collaborative project to create a free editable map of the world. This is crowd-sourced open source alternative to Google Maps. For example, the following institutions use it instead of Google Maps:

• Craigslist
• Apple
• Foursquare

Leaflet is the leading open-source JavaScript library for mobile-friendly interactive maps.

Quote on their page: Leaflet doesn't try to do everything for everyone. Instead it focuses on making the basic things work perfectly.

Example: Classics Major Haley Tilt at Reed's project on Livy's Rome.

There is an R package to interface with Leaflet which simply hooks Leaflet into R: https://rstudio.github.io/leaflet/

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.

If you understand ggplot(), you can understand leaflet():

• It sets the base layer, to which we add layers.
• You can specify data, which can be
  • a regular data frame with columns
    • lat or latitude
    • lng, long, lng, or longitude
  • a maps package data frame
  • an sp package object. See this link

The sp package is the go-to package for vector data (as opposed to raster) for GIS in R. Objects can be either

• SpatialPoints
• SpatialLines
• SpatialPolygons

For example, when you load a shapefile into R using rgdal::readOGR, it is loaded as a sp package object.

• Can embed in R Markdown documents
• Can integrate with Shiny