Making maps

Maps are cool and maps are useful, but I always here people say that making maps is hard. I think making maps is simpler today than ever before. So to show you what I mean, this post is how to map an open spatial dataset.

Get it

Geojson formatted data is available in your local data portal, I promise.

I’m using Census tract data hosted on Charlottesville’s OpenData portal.

library(sf)

## Linking to GEOS 3.7.2, GDAL 2.4.2, PROJ 5.2.0

tracts <- read_sf("https://opendata.arcgis.com/datasets/63f965c73ddf46429befe1132f7f06e2_15.geojson")

Wrangle it

This step is the most variable depending on what you have to do. But this data set comes formatted pretty well, so I don’t really have to do much.

I set up a numeric column with the fraction of residents who are black for my fill scale and a character column with text for a tooltip.

library(dplyr)

## 
## Attaching package: 'dplyr'

## The following objects are masked from 'package:stats':
## 
##     filter, lag

## The following objects are masked from 'package:base':
## 
##     intersect, setdiff, setequal, union

tracts <- tracts %>% 
  mutate(frac_black = round(Black / Population, 2),
         tooltip_text = glue::glue("{frac_black * 100} % of residents are black"))

Map it

Leaflet has been around forever and is used everywhere on the web. There are lots of new mapping packages coming out for R, but I always go back to Leaflet becasue I think the easiest to use.

Leaflet requires scales be defined indendependly so I create a scale to fill with first. Then I pass in column names to polygon arguments, with the ~ syntax, which is very simlar to the aes syntax of ggplot2.

library(leaflet)

black_pal <- colorNumeric("Greys", tracts$frac_black)

leaflet(tracts) %>% 
  addPolygons(fillColor = ~black_pal(frac_black),
              label = ~tooltip_text,
              color = "black", fillOpacity = 1) %>% 
  addLegend(pal = black_pal, values = ~frac_black,
            title = "Proportion black (2010):",
            opacity = 1) %>% 
  widgetframe::frameWidget(height = '400') # to fix blog CSS conflicts; you probably don't need this line

Wrap up

Maps are super powerful for sharing info about communities. I hope after reading this you feel like you can make a map with Rstats. And I hope you use it visualize some data about where you live.