Cartography Example

MEDS

data visualization

A brief example showing how to use cartography to add a hatched layer to a map.

Author

Affiliation

Jessica French

MEDS

Published

December 20, 2022

Introduction

The cartography package can be used with base r plot to add map elements and improve the legibility of your maps. Hatching can be an effective way to visualize a map feature but can be a little complicated to add. In this example we will use US States data from the spData package and the cartography package to show how you can add hatching to a map.

# spData makes the us_states data frame available. 
library(spData)
# sf is needed to read the geometry in the us_states data frame which is an sf object. 
library(sf)
# cartography allows you to add the hatched and choropleth map layers. 
library(cartography)
# adds functionality the helps us rearrange the data and is generally wonderful. 
library(tidyverse)

Lets visualize population growth in the United States between 2010 and 2015 with a hatched feature showing states that experienced negative growth.

# Use mutate from the tidyverse to create a new variable for population growth called pop_growth.
us_states_diff <- us_states |> 
  mutate("pop_growth" = total_pop_15 - total_pop_10)

In order to add the hatched layer we will need a data frame with only the polygons that will be hatched. We can use the tidyverse to accomplish this.

# Creates a data frame with only polygons of states that had negative population growth. 
hatched_df <- us_states_diff |> 
  select(pop_growth) |> 
  filter(pop_growth < 0)

The cartography package creates layers that can be plotted using plot from base r. This requires that we create a base plot and add all of the layers we want in one code chunk. Here is a quick preview of what we will do below: - Create a base plot. - Create and add a choropleth layer of population growth. - Create and add a hatched layer of showing states with negative population growth. - Add a legend - Set layout options.

# Creates the base plot that all the following layers will be plotted on top of. 
plot(us_states$geometry)

###############################
####Choropleth Layers##########
###############################

# Creates and add the choropleth layer to the base plot
choroLayer(
  # specify the data.
  x = us_states_diff,
  # specify the variable to be plotted.
  var = "pop_growth",
  # specify the method of creating breaks.
  method = "jenks",
  # specify the number of classes.
  nclass = 5,
  # sepecify the color palette, these need to be in the order you want them to appear.
  col = c("#cce7c9","#8bca84","#5bb450","#46923c","#276221"),
  # specify border color.
  border = "black",
  # specify line weight.
  lwd = 0.5,
  # sets legend position.
  legend.pos = "bottomright",
  # sets legend title size.
  legend.title.cex = 0.5,
  # sets legend values size.
  legend.values.cex = 0.5,
  # sets legend title.
  legend.title.txt = "Population \nGrowth",
  # adds a frame to the legend.
  legend.frame = TRUE,
  # adds this layer to the previous plot.
  add = TRUE)

# This layer is purely aesthetic and makes it so that the map of United States apears on top of the legend created in the previous layer. 
choroLayer(
  x = us_states_diff,
  var = "pop_growth",
  method = "jenks",
  nclass = 5,
  col = c("#cce7c9","#8bca84","#5bb450","#46923c","#276221"),
  border = "black",
  lwd = 0.5,
  legend.pos = "n",
  add = TRUE)

###############################
###HATCHED LAYER###############
###############################

# Creates the hatched layer and adds it to the plot.  
hatchedLayer(
  # the data to be plotted, this is the data frame we created earlier of only the states to the hatched. 
  x = hatched_df,
  # sets the pattern
  pattern = "diamond",
  # sets the densiry of the pattern,. 
  density = 4,
  # sets the line weight. 
  lwd = 0.3,
  # adds the layer to the previous plot. 
  add = TRUE)

#########################################
######Hatched Legend#####################
#########################################

# Creates a legend for the hatched layer. This will not be part of the choropleth legend. 
legendHatched(pos = "bottomleft",
              title.txt = "",
              categ = "States with Negative Population Growth",
              patterns = "diamond",
              density = 1,
              col = "black",
              ptrn.bg = "white")

###############################
#######Layout options##########
###############################
# Creates a layout layer for displaying the map. 
layoutLayer(title = "Population Growth in the United States",
            frame = TRUE,
            scale = FALSE
             )

Citation

BibTeX citation:

@online{french2022,
  author = {Jessica French},
  title = {Cartography {Example}},
  date = {2022-12-20},
  url = {https://jessicafrench.github.io/code_examples/2022-12-20-cartography-example},
  langid = {en}
}

For attribution, please cite this work as:

Jessica French. 2022. “Cartography Example.” December 20, 2022. https://jessicafrench.github.io/code_examples/2022-12-20-cartography-example.