Example of a geospatial union between polygons with R's {sf} package

union-polygons-sf.R

# Example of a geospatial union between polygons with R's {sf} package
# Combine East and West Midlands to 'Midlands'

# This example uses a GeoJSON of super-generalised clipped boundaries for England's regions:
# http://geoportal.statistics.gov.uk/datasets/regions-december-2017-super-generalised-clipped-boundaries-in-england
# (click the API dropdown on this page and copy the URL for the GeoJSON)

# Packages
library(geojsonio)
library(sf)
library(ggplot2)

# Read GeoJSON file
region_json <- geojson_read(
  x = "https://opendata.arcgis.com/datasets/4fcca2a47fed4bfaa1793015a18537ac_3.geojson",
  what = "sp"  # spatial class
)

# Create column that's TRUE when it's the midlands
region_sf <- st_as_sf(region_json) %>%  # convert to {sf} format
  mutate(midlands = ifelse(rgn17nm %in% c("East Midlands", "West Midlands"), "TRUE", FALSE))

# Create {sf} object that's all regions wihtout E and W Midlands
not_midlands <- region_sf %>%
  filter(midlands == FALSE) %>% 
  select(region = rgn17nm, geometry) # we only need the region name and geometry

# Create {sf} object that's just the E and W Midlands
midlands <- region_sf[region_sf$midlands == TRUE, ] %>% 
  st_union() %>%  # union of the E and W Midlands polygons
  st_sf() %>%  # back to {sf} object
  mutate(region = "Midlands")  # give the new polygon a name

# Bind the two objects 
regions_simplified <- rbind(not_midlands, midlands)