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

问题

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" )

回答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

来源：https://stackoverflow.com/questions/49354393/r-how-do-i-merge-polygon-features-in-a-shapefile-with-many-polygons-reproducib

标签

merge

polygon

shapefile