Creating Hexbin Maps in R
Contents
Objective
The objective of this tutorial is to create a hexbin choropleth map of U.S. education costs in RStudio. Users will learn how to create a hexbin map from a geospatial object and plot thematic data. In addition, this tutorial will demonstrate how to add and customize various cartographic elements to your map including symbolization, labelling, and map elements (e.g., title, legend). Users will have the opportunity to become more familiar with the R programming language as well as explore the spatial and cartographic capabilites of the software. This tutorial uses open-source software and data and will discuss the advantages and limitations of each. Finally, this tutorial contributes to the collection of Open Source GIS tutorials created by students at Carleton University.
Software Requirements: R, RStudio, spreadsheet software (e.g., Microsoft Excel) Skills:
Note: This tutorial assumes basic knowledge of the R programming language. This version of the tutorial was created using a Windows platform with R version 3.6.2.
Why Hexagons?
Regularly shaped grids are often used to normalize geography for mapping in instances where polygons are irregularly shaped (i.e., political boundaries). A hexagon grid is an alternative to the square (fishnet) grid typically used in GIS analysis and thematic mapping. Aggregating data into hexagons is advantageous as the edge effects of the grid shape reduce sampling bias. In addition, hexagons can be used to obscure sensitive source data (e.g., personal addresses).
Getting Started
Downloading the Software
The first step of this tutorial is downloading R and RStudio if they are not already installed on your device. R version 4.1.2 is the latest version of the software released in 2021.
R is a widely used open-source software environment used for data manipulation and analysis (statistics, graphics, etc.). R is easily customizable and is executed line by line in a console. For the purposes of this tutorial, R will be accessed through an integrated development environment (IDE) called RStudio.
Finding Data
Spatial data
When creating a hexagonal map, users have the option to create a hexbin map from (1) a geospatial object or (2) a list of coordinates. For the purposes of this tutorial, we will use an existing hexagon boundary file (.geojson) of the United States. Download the data in .geojson format and save to a new project folder.
The hexgrid is available to download HERE.
Non-spatial data
The statistical data for this tutorial will be sourced from the United States (U.S.) National Science Board. The data of interest is the state-level “Average Undergraduate Charge at Public 4-Year Institutions” from 1994 to 2019. The charge includes the tuition, required fees, room, and board for a full-time undergraduate student who is a state resident. This data serves as a useful indicator of the accessibility of higher education.
The data is available to view and download HERE.
The RStudio workspace consists of four panes:
- (Top left): This is the code editor
- (Bottom left): This is the R console
- (Top right):
- (Bottom right):
To start a new R script, click the icon in the top left corner of the script window or by clicking through the top menu (File > New > R script).
Next, save the R script to the folder that contains your data from the step above. Click the 'save' icon in the script window or (File > Save As).
Set your working directory at the start of your new session. This is the folder where R reads and saves files. This can be accessed through the top menu (Session > Set Working Directory > Choose Directory) or by writing a command:
setwd("~/FALL 2021/GEOM 4008/Data")
Creating a Choropleth Map
Installing Packages
This tutorial requires a number of packages (see list below). These packages can be installed using the top menu (Tools > Install Packages).
Tip: If you want to learn more about a specific package or function, you can write a command in the R console to the view the corresponding 'Help' page.
?mutate
Importing Data
Before reading your data into R, ensure the files are located in the folder you set as your directory.
To import the hexbin data, we will use the geojson_read() function. Once imported, the data will appear in the 'Global Environment' tab in the top-right pane.
hex <- geojson_read("us_states_hexgrid.geojson", what = "sp")
After the file has been imported, we will need to reformat and fortify the data using the mutate and gsub in order to perform a spatial join and plot our map.
#Reformat the 'google_name' field
#This will remove the (United States) from each value
#E.g., Vermont (United States) will be changed to Vermont
hex@data = hex@data %>% mutate(google_name = gsub(" \\(United States\\)", "", google_name))
Fortify the data using the tidy function. This will produce a data frame output which is necessary to plot the data using the ggplot2 library.
hex_fortify <- tidy(hex, region = "google_name")
Join Spatial and Non-Spatial Data
To create our choropleth map, we will need to join our spatial and non-spatial data together. To perform the join, we can write a command specifying the two fields that will be used. Using the fortified hexbin data, we will join the field "id" with the "state" field from the data table.
hex_fortify <- hex_fortify %>%
left_join(. , undergrad, by=c("id"="State"))
Symbolization
Add Map Elements
Labels can be added to our map to provide viewers with geographic reference information. This is especially important on our hexbin map where the U.S. states boundaries are not shown as they would appear on a political map. To add labels to the hexbin map, we must first calculate the centroid of each hexagon using the gCentroid function. We will use the two-letter state abbreviations in the "id" field. The labels will be added to the plot and we are then able to change their colour and size.
Conclusion
References
Holtz, Yan. (n.d.). Hexbin map in R: an example with US states. https://www.r-graph-gallery.com/328-hexbin-map-of-the-usa.html
https://team.carto.com/u/andrew/tables/andrew.us_states_hexgrid/public/map
Esri. (2015, April 8). Thematic mapping with hexagons. https://www.esri.com/about/newsroom/insider/thematic-mapping-with-hexagons/
