# Joining polygons in R

I am wondering how to join spatial polygons using R code?

I'm working with census data where certain areas change over time and I wish to join the polygons and the corresponding data and simply report on the joined areas. I am maintaining a list of polygons that have changes census to census and that I plan to merge. I'd like to use this list of area names as a lookup list to apply to census data from different years.

I'm wondering what R function to use to merge selected polygons and respective data. I have googled it but simply become confused by results.

• The answer to most geometry operations like polygon dissolving, overlay, point-in-polygon, intersection, union etc etc is the rgeos package. Jun 15, 2013 at 7:44
• The US Census Bureau publishes tables to do this for 1990-2000 and 2000-2010. They can be managed with database joins, which are implemented by R's merge function. Jun 15, 2013 at 16:56

The following solution is based on a post by Roger Bivand on R-sig-Geo. I took his example replacing the German shapefile with some census data from Oregon you can download from here (take all shapefile components from 'Oregon counties and census data').

# Required packages
libs <- c("rgdal", "maptools", "gridExtra")
lapply(libs, require, character.only = TRUE)

# Import Oregon census data
oregon <- readOGR(dsn = "path/to/data", layer = "orcounty")
oregon.coords <- coordinates(oregon)

Next, you need some grouping variable in order to aggregate the data. In our example, grouping is simply based on the single county coordinates. See the image below, black borders indicate the original polygons, whereas red borders represent polygons aggregated by oregon.id.

# Generate IDs for grouping
oregon.id <- cut(oregon.coords[,1], quantile(oregon.coords[,1]), include.lowest=TRUE)

# Merge polygons by ID
oregon.union <- unionSpatialPolygons(oregon, oregon.id)

# Plotting
plot(oregon)
plot(oregon.union, add = TRUE, border = "red", lwd = 2)

So far, so good. However, data attributes related to the original shapefile's subregions (e.g. population density, area, etc.) get lost when performing unionSpatialPolygons. I guess you'd like to aggregate your census data associated to the shapefile as well, so you'll need an intermediate step.

You first have to convert your polygons to a dataframe in order to perform aggregation. Now let's take data attribute columns six to eight ("AREA", "POP1990", "POP1997") and aggregate them according to the above IDs applying function sum.

# Convert SpatialPolygons to data frame
oregon.df <- as(oregon, "data.frame")

# Aggregate and sum desired data attributes by ID list
oregon.df.agg <- aggregate(oregon.df[, 6:8], list(oregon.id), sum)
row.names(oregon.df.agg) <- as.character(oregon.df.agg\$Group.1)

Finally, reconvert your dataframe back to a SpatialPolygonsDataFrame providing the previously unified shapefile oregon.union and you obtain both generalized polygons and your census data derived from above summarization aggregation step.

# Reconvert data frame to SpatialPolygons
oregon.shp.agg <- SpatialPolygonsDataFrame(oregon.union, oregon.df.agg)

# Plotting
grid.arrange(spplot(oregon, "AREA", main = "Oregon: original county area"),
spplot(oregon.shp.agg, "AREA", main = "Oregon: aggregated county area"), ncol = 1)

• the link to your Oregon shapefile seems to be dead and I have a hard time loading in other shapefiles from Oregon Jul 17, 2020 at 14:22

Here is a solution using the sf package:

library(tidycensus)
library(dplyr)
library(sf)
library(ggplot2)

# get data from tindycensus for demonstration (note you need an API key, folow instructions here: https://walkerke.github.io/tidycensus/articles/basic-usage.html)
census <- tidycensus::get_acs(geography = "tract", variables = "B19013_001",
state = "TX", county = "Tarrant", geometry = TRUE) %>%
arrange(NAME)

# reduce dataset size
census <- census[1:8,]

# create grouping variable
group_1 <- census\$GEOID[1:2]
group_2 <- census\$GEOID[6:8]

census <- census %>% mutate(group = case_when(GEOID %in% group_1 ~ 'newgroup1',
GEOID %in% group_2 ~ 'newgroup2',
TRUE ~ GEOID))

# summarise by grouping variable (performs a union on grouped polygons and sums 'estimate')
census2 <- group_by(census, group) %>%
summarise(estimate = sum(estimate), do_union = TRUE)

# visualise using ggplot2 development version and facet by merged/unmerged datasets
plot_data <- rbind(census %>% select(group, estimate) %>%
mutate(facet = "unmerged"),
census2 %>% mutate(facet = "merged"))

gp <- ggplot() +
geom_sf(data = plot_data, aes(fill = estimate), color = 'white') +
scale_fill_viridis_c() +
facet_wrap(~facet, ncol = 1)

• I thought I'd just add a little warning here, just in case: beware of using summarise() derivatives with the do_union argument, as I just did something like summarise_if(shapefile, predic.function, sum, na.rm = TRUE, do_union = TRUE), which ended up also summing a TRUE in each cell (i.e. +1 for all operations). Need to investigate more to figure out if that's something that should be reported (at least for an extra warning)...? Jul 5, 2019 at 1:25