Simple way to subset SpatialPolygonsDataFrame (i.e. delete polygons) by attribute in R
I would like simply delete some polygons from a SpatialPolygonsDataFrame object based on corresponding attribute values in the @data data frame so that I can plot a simplified/subsetted shapefile. So far I haven't found a way to do this.
For example, let's say I want to delete all polygons from this world shapefile that have an area of less than 30000. How would I go about doing this?
Or, similarly, how can I delete Antartica?
require(maptools)
getinfo.shape("TM_WORLD_BORDERS_SIMPL-0.3.shp")
# Shapefile type: Polygon, (5), # of Shapes: 246
world.map <- readShapeSpatial("TM_WORLD_BORDERS_SIMPL-0.3.shp")
class(world.map)
# [1] "SpatialPolygonsDataFrame"
# attr(,"package")
# [1] "sp"
head(world.map@data)
# FIPS ISO2 ISO3 UN NAME AREA POP2005 REGION SUBREGION LON LAT
# 0 AC AG ATG 28 Antigua and Barbuda 44 83039 19 29 -61.783 17.078
# 1 AG DZ DZA 12 Algeria 238174 32854159 2 15 2.632 28.163
# 2 AJ AZ AZE 31 Azerbaijan 8260 8352021 142 145 47.395 40.430
# 3 AL AL ALB 8 Albania 2740 3153731 150 39 20.068 41.143
# 4 AM AM ARM 51 Armenia 2820 3017661 142 145 44.563 40.534
# 5 AO AO AGO 24 Angola 124670 16095214 2 17 17.544 -12.296
If I do something like this, the plot does not reflect any changes.
world.map@data = world.map@data[world.map@data$AREA > 30000,]
plot(world.map)
same result if I do this:
world.map@data = world.map@data[world.map@data$NAME != "Antarctica",]
plot(world.map)
Any help is appreciated!
Answer
looks like you're overwriting the data, but not removing the polygons. If you want to cut down the dataset including both data and polygons, try e.g.
world.map <- world.map[world.map$AREA > 30000,]
plot(world.map)
[[Edit 19 April, 2016]]
That solution used to work, but @Bonnie reports otherwise for a newer R version (though perhaps the data has changed too?):
world.map <- world.map[world.map@data$AREA > 30000, ]
Upvote @Bonnie's answer if that helped.