mapping - Simple way to subset SpatialPolygonsDataFrame (i.e. delete polygons) by attribute in R

Question

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 1

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.