ggplot omits polygon holes
ggplot2, r
Solution
"hadley/ggplot2: plotting polygon shapefiles" says "..., the hole state is retained, but this information is not used by ggplot2". Fortunately, `ggspatial` package, "Fish & Whistle: Holes in ggplot polygons" taught me, seem to solve some problems, such as the overlapping problem. (The white area upper the label of "Biggin Hill" isn't due to `ggspatial` package because it is also on `ggplot(d) + geom_polygon(aes(long, lat, group = group, fill = id))` on my env)
devtools::install_github("paleolimbot/ggspatial")
library(ggspatial)
ggmap(toner) + geom_spatial(data=v, aes(fill=id), alpha=.8)
Problem
I'm having difficulty getting ggplot to plot polygons that feature holes. The following demonstrates. First get the shapefile with `git clone https://github.com/geotheory/volcano`. Next: ``` require(ggplot2); require(ggmap); require(dplyr); require(maptools) v = readShapePoly('volcano/volcano.shp') v@proj4string = CRS('+proj=longlat +datum=WGS84') # confirm polygons spatially exclusive (don't overlap) plot(t(bbox(v)), type='l', lwd=8) plot(v, col=paste0(colorRampPalette(c('grey','red'))(8),'dd'), add=T) ``` Looks ok. A `dd` alpha should render the line invisible if obscured by multiple polygons. Now let's try in ggplot. ``` d = fortify(v) %>% as_data_frame() bb = bbox(v) toner = get_stamenmap(c(bb[1,1], bb[2,1], bb[1,2], bb[2,2]), zoom=11, maptype='toner') ggmap(toner) + geom_polygon(data=d, aes(long, lat, group=group, fill=id), alpha=.5) ``` Central polygons must be overlapping since the underlying map is completely obscured in the centre. Let's check the fortified data for holes: ``` d %>% select(id, hole) %>% table() hole id FALSE TRUE 0 278 0 1 715 0 2 392 388 3 388 331 4 390 265 5 265 387 6 328 125 7 125 0 ``` Looks ok, so let's try visualising them individually. ``` i = 3 plot(v[i,], col='red') ggplot(filter(d, id == i-1)) + geom_polygon(aes(long, lat, group=group, col=hole), fill=NA) ggplot() + geom_polygon(data=filter(d, id==i-1), aes(long,lat, group=group)) ``` Something appears to be going wrong. ggplot seems to be ignoring the holes. Unless it's a problem with the shapefile. Any suggestions how to diagnose/fix this?