Remove lines from inside a polygon (shapefile) in r

Question

I have this code which prints the Galápagos Islands. But inside the islands, there are lines that I want to get rid off. How is this possible in R?

library(rgdal)
library(ggplot2)

URL <- "https://osm2.cartodb.com/api/v2/sql?filename=public.galapagos_islands&q=select+*+from+public.galapagos_islands&format=geojson&bounds=&api_key="
fil <- "gal.json"
if (!file.exists(fil)) download.file(URL, fil)
gal <- readOGR(fil, "OGRGeoJSON")    
gal_map <- fortify(gal)
ggplot() + 
  geom_map(map=gal_map, data=gal_map, aes(x=long, y=lat, map_id=id)) + 
  coord_equal()

Look at the picture: I don't want to show these lines (all of them).

Answer 1

You can aggregate the polygons and then disaggregate to the 182 individual islands.

URL <- "https://osm2.cartodb.com/api/v2/sql?filename=public.galapagos_islands&q=select+*+from+public.galapagos_islands&format=geojson&bounds=&api_key="
fil <- "gal.json"
if (!file.exists(fil)) download.file(URL, fil)

library(rgdal)
gal <- readOGR(fil, "OGRGeoJSON")   

library(raster)
library(rgeos)

a <- aggregate(gal)
b <- disaggregate(a)

plot(b, col=sample(rainbow(length(b))))

Answer 2

I assume that the 'lines' you are referring to are simply the polygon boundaries separating one feature from another. If you just want to display the data and you do not necessarily have to stick with ggplot2, a possible option would be to use spplot. Just create a plot of the non-unified polygons with "transparent" line color (see ?sp.polygons) and add the island boundary afterwards using layer from latticeExtra. Note the use of unionSpatialPolygons from maptools to merge all polygons into one, thus ending up with the coastline only.

## add 'id' column
gal@data$id <- seq(gal@data$fid) - 1

## unify polygons
library(maptools)
gal_union <- unionSpatialPolygons(gal, IDs = rep(1, length(gal)))

## display data with island boundaries only
library(latticeExtra)
spplot(gal, "id", col = "transparent", scales = list(draw = TRUE), 
       at = seq(gal@data$id), col.regions = rainbow(nrow(gal@data))) + 
  layer(sp.polygons(gal_union, fill = "transparent"))

If I got you wrong and you are trying to merge particular polygons e.g. based on their spatial location, I highly recommend having a deeper look at ?unionSpatialPolygons and, for instance, the related post on Joining polygons in R.

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Update:

This would be a possible (yet very basic) ggplot2 version of the above code (with the legend disabled).

gal_ff <- fortify(gal)
gal_union_ff <- fortify(gal_union)

ggplot(aes(x = long, y = lat, group = id, fill = id), data = gal_ff) + 
  geom_polygon() + 
  geom_path(aes(x = long, y = lat, group = group), data = gal_union_ff, 
            colour = "black") +
  scale_fill_discrete(guide = FALSE) + 
  coord_equal()