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A user recently asked me how to add a state outline to a Zip Code map. Consider this map, which shows the percent white of each Zip Code Tabulated Area (ZCTA) in New York and New Jersey:

Clearly the ZCTAs in northern New York have the highest concentration of white residents. However, it would still be useful to see the border between New York and New Jersey.

### Step 1: Build the Initial Map

This map has one complexity to it: it hides the actual ZCTA borders. This technique is very useful when viewing a large number of Zip codes and is described in detail here.

```# Load the necessary packages
library(choroplethr)
library(ggplot2)
library(devtools)
install_github('arilamstein/[email protected]')
library(choroplethrZip)

# load the data
data(df_zip_demographics)
df_zip_demographics\$value = df_zip_demographics\$percent_white

# create the map
zip_map = ZipChoropleth\$new(df_zip_demographics)
zip_map\$ggplot_polygon = geom_polygon(aes(fill = value),
color = NA)
zip_map\$set_zoom_zip(state_zoom  = c("new york", "new jersey"),
county_zoom = NULL,
msa_zoom    = NULL,
zip_zoom    = NULL)
zip_map\$title = "New York and New Jersey ZCTAs"
zip_map\$legend = "Percent White"
zip_map\$set_num_colors(4)
choro = zip_map\$render()
choro
```

### Step 2: Create the State Outlines

State outlines are rendered in the render_state_outline function in the USAChoropleth object. Here is how to store the outline:

```data(df_pop_state)
outline = StateChoropleth\$new(df_pop_state)
outline = outline\$render_state_outline(c("new york", "new jersey"))
```

### Step 3: Render the Combined Maps

The zip map and the outline are both ggplot objects. This means that they can be combined with the + sign:

```choro_with_outline = choro + outline
choro_with_outline
```

The post User Question: How to Add a State Border to a Zip Code Map? appeared first on AriLamstein.com.