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.

Answer 2

When I tried to do this in R 3.2.1, tim riffe's technique above did not work for me, although modifying it slightly fixed the problem. I found that I had to specifically reference the data slot as well before specifying the attribute to subset on, as below:

world.map <- world.map[world.map@data$AREA > 30000, ] plot(world.map) 

Adding this as an alternative answer in case others come across the same issue.