Choroplethr version 3.6.0 is now on CRAN. This version adds functionality for getting and mapping demographics of US Census Tracts. You can install it from the R console as follows:
install.packages("choroplethr")
packageVersion("choroplethr")
[1] ‘3.6.0’
To use this functionality you will need an API key from the US Census Bureau. You can learn more about that here.
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Getting Tract Maps
Historically choroplethr has had limited support for Census Tracts. This is because the US Census Bureau releases tract maps on a per-state basis, and it wasn’t feasible to create a separate package for each state.
Choroplethr now uses the tigris package to download Tract maps from the Census Bureau on demand. The function to retrieve a map is get_tract_map. Here’s an example of retrieving and rendering a map of Tracts in New York:
library(choroplethr)
library(ggplot2)
?get_tract_map
ny = get_tract_map("new york")
ggplot(ny, aes(long, lat, group=group)) + geom_polygon()
Getting Tract-Level Demographics
Choroplethr contains both maps and interesting data to map. If you want to explore the demographics of US Census Tracts then use get_tract_demographics:
# see help for extra options
?get_tract_demographics
ny_stats = get_tract_demographics("new york")
head(ny_stats)
region total_population percent_white percent_black percent_asian percent_hispanic per_capita_income median_rent median_age 36001000100 2163 19 57 2 19 19065 596 36.9 36001000200 5335 9 72 0 13 15376 501 27.8 36001000300 6077 35 44 3 17 20804 743 31.0 36001000401 2380 88 7 2 2 39574 1198 65.5 36001000403 4338 65 19 11 5 32397 859 41.9 36001000404 4932 69 12 7 9 2479 NA 19.6
Creating Choropleth Maps
Now that we have a map and spatial data, we can create a choropleth map with the function tract_choropleth.
Recall that all choroplethr functions require a dataframe where one column is called “region” and one column is called “value”. get_tract_demographics returns a dataframe with a “region” column and eight demographic values. We still need to create a “value” column. Let’s go with median_rent:
ny_stats$value = ny_stats$median_rent ?tract_choropleth tract_choropleth(ny_stats, "new york", title = "2013 Median Rent\nCensus Tracts", legend="Dollars")
People not familiar with New York might see the above map and not know where major landmarks are. To solve this problem, all choroplethr functions have a “reference_map” parameter, which puts a google map underneath the choropleth:
tract_choropleth(ny_stats, "new york", title = "2013 Median Rent\nCensus Tracts", legend="Dollars", reference_map = TRUE)
Zooming in
Tract maps of an entire state are hard to view because the tracts are so small. This is why all tract-related functions in choroplethr allow you to zoom by county.
In addition to being useful for viewing maps, the county-zoom option is useful for get_tract_demographics because getting tract-level demographics for an entire state is slow.
Note that counties must be specified by their county FIPS code. Here’s an example of zooming in on Manhattan (FIPS code 36061):
# 36061 is the FIPS code for New York county (i.e. Manhattan)
manhattan_2010 = get_tract_demographics("new york", county_fips=36061, endyear = 2010, span = 5)
manhattan_2010$value = manhattan_2010$median_rent
m1 = tract_choropleth(manhattan_2010, "new york", legend = "Dollars", county_zoom = 36061)
m2 = tract_choropleth(manhattan_2010, "new york", legend = "Dollars", county_zoom = 36061, reference_map = TRUE)
?double_map
double_map(m1, m2, "2010 Median Rent\nManhattan Census Tracts")
New function: double_map
As the above example shows, v3.6.0 introduces a new function: double_map.
In my own work I find that frequently switch between viewing a pure choropleth (where the color contrast is strong) and a choropleth + reference map (where the reference map helps you understand what you’re looking at). double_map simply encapsulates some code for putting two maps side-by-side
Caveats
There are a few things to keep in mind when using this functionality:
- get_tract_demographics returns data from the American Community Survey (ACS). The ACS provides estimates with a margin of error (MOE). In the case of tracts, the MOE can be quite large. tract_choropleth does not attempt to visualize the MOE.
- get_tract_demographics sometimes produces the warning “NAs introduced by coercion”. I assume that this occurs when the ACS simply does not return an estimated value for a tract. (E.g. a sparsely populated, or unpopulated tract). However, I have not investigated this.
- tract_choropleth sometimes produces the warning “Your data.frame contains the following regions which are not mappable”. This is a bit of a mystery for me. It indicates that get_tract_demographics (which gets data from the Census API) contains regions which are not part of a map (that also comes from the Census API).
- tract_choropleth sometimes produces the warning “The following regions were missing and are being set to NA”. Presumably these regions are the same regions as in (2).
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