R How do I merge polygon features in a shapefile with many polygons? (reproducible code example)

Question

How do I merge polygon features in a shapefile with many polygons?

rbind and union just combine the rows of the shapefile features, they don't actually merge the polygons themselves.

Desired result in example below:

How do I get the below shapefile with duplicated ID_2 to merge to a single polygon in sptemp?

Example below of GADM level 2 of Ethiopia has first two rows of the shapefile ID_2 column duplicated (value=1). I'd like sptemp with 79 features combining the first two rows that are the ones with duplicated ID_2. A plot of sptemp[1,] would bring up where current sptemp[1,] and sptemp2[2,] without the boundaries between the duplicated, i.e. the polygons are merged too.

Example Code:

Download, unzip, and load into R GADM file for Ethiopia level 2 (899kb to working directory):

library(curl)
library(rgdal)

curl_download("http://biogeo.ucdavis.edu/data/gadm2.8/shp/ETH_adm_shp.zip",
              destfile=paste0("gadmETH.zip"),
              quiet=FALSE)

unzip(paste0("gadmETH.zip"), exdir="gadmETH", overwrite=FALSE) 


###Load shapefile
sptemp <- readOGR(dsn="gadmETH", layer="ETH_adm2")

The ID_2 column for the first two polygons is duplicated

###You'll see in the first two rows ID_2 is duplicated
df.sptemp <- as.data.frame(sptemp)

View(sptemp)

###I can't just delete one because they are separate polygons 
plot(sptemp[1,], col="blue")
plot(sptemp[2,], add=TRUE, col="red" )

Answer 1

Note This method uses st_union, which combines all the 'multipolygons' into single polygons. This may not be your actual desired result.

---

If you use library(sf) as opposed to sp (it's the successor to sp), you can use st_union to join geometries.

You can do this inside a dplyr pipe sequence too.

library(sf)
sptemp <- sf::st_read(dsn = "~/Desktop/gadmETH/", layer = "ETH_adm2")

library(dplyr)

sptemp %>% 
    group_by(ID_2) %>%
    summarise(geometry = sf::st_union(geometry)) %>%
    ungroup()

# Simple feature collection with 79 features and 1 field
# geometry type:  GEOMETRY
# dimension:      XY
# bbox:           xmin: 33.00154 ymin: 3.398823 xmax: 47.95823 ymax: 14.84548
# epsg (SRID):    4326
# proj4string:    +proj=longlat +datum=WGS84 +no_defs
# # A tibble: 79 x 2
#      ID_2                       geometry
#     <dbl>         <sf_geometry [degree]>
#   1    1. POLYGON ((38.85556 8.938293...
#   2    2. POLYGON ((42.15579 12.72123...
#   3    3. POLYGON ((40.17299 14.49028...
#   4    4. POLYGON ((41.11739 10.93207...
#   5    5. POLYGON ((40.61546 12.7958,...
#   6    6. POLYGON ((40.25209 11.24655...
#   7    7. POLYGON ((36.35452 12.04507...
#   8    8. POLYGON ((40.11263 10.87277...
#   9    9. POLYGON ((37.39736 11.60206...
#   10   10. POLYGON ((38.48427 12.32812...
#  # ... with 69 more rows