Plot spatial area defined by multiple polygons

Question

I have a SpatialPolygonsDataFrame with 11589 spatial objects of class "polygons". 10699 of those objects consists of exactly 1 polygon. However, the rest of those spatial objects consist of multiple polygons (2 to 22).

If an object of consists of multiple polygons, three scenarios are possible:

1) The additional polygons could describe a "hole" in the spatial area described by the first polygon . 2) The additional polygons could also describe additional geographic areas, i.e. the shape of the region is quite complex and described by putting together multiple parts. 3) Often it is a mix of both, 1) and 2).

My question is: How to plot such a spatial object which is based on multiple polygons?

I have been able to extract and plot the information of the first polygon, but I have not figured out how plot all polygons of such a complex spatial object at once.

Below you find my code. The problem is the 15th last line.

# Load packages
# ---------------------------------------------------------------------------
library(maptools)
library(rgdal)
library(ggmap)
library(rgeos)


# Get data
# ---------------------------------------------------------------------------
# Download shape information from the internet
URL <- "http://www.geodatenzentrum.de/auftrag1/archiv/vektor/vg250_ebenen/2012/vg250_2012-01-01.utm32s.shape.ebenen.zip"
td <- tempdir()
setwd(td)
temp <- tempfile(fileext = ".zip")
download.file(URL, temp)
unzip(temp)

# Get shape file
shp <- file.path(tempdir(),"vg250_0101.utm32s.shape.ebenen/vg250_ebenen/vg250_gem.shp")

# Read in shape file
x <- readShapeSpatial(shp, proj4string = CRS("+init=epsg:25832"))

# Transform the geocoding from UTM to Longitude/Latitude
x <- spTransform(x, CRS("+proj=longlat +datum=WGS84"))

# Extract relevant information 
att <- attributes(x)
Poly <- att$polygons


# Pick an geographic area which consists of multiple polygons
# ---------------------------------------------------------------------------
# Output a frequency table of areas with N polygons 
order.of.polygons.in.shp <- sapply(x@polygons, function(x) x@plotOrder)
length.vector <- unlist(lapply(order.of.polygons.in.shp, length))
table(length.vector) 

# Get geographic area with the most polygons
polygon.with.max.polygons <- which(length.vector==max(length.vector))
# Check polygon
# x@polygons[polygon.with.max.polygons]

# Get shape for the geographic area with the most polygons
### HERE IS THE PROBLEM ###
### ONLY information for the first polygon is extracted ###
Poly.coords <- data.frame(slot(Poly[[polygon.with.max.polygons ]]@Polygons[[1]], "coords"))

# Plot
# ---------------------------------------------------------------------------
## Calculate centroid for the first polygon of the specified area
coordinates(Poly.coords) <- ~X1+X2
proj4string(Poly.coords) <- CRS("+proj=longlat +datum=WGS84")
center <- gCentroid(Poly.coords)

# Download a map which is centered around this centroid
al1 = get_map(location = c(lon=center@coords[1], lat=center@coords[2]), zoom = 10, maptype = 'roadmap')

# Plot map
ggmap(al1) + 
  geom_path(data=as.data.frame(Poly.coords), aes(x=X1, y=X2))

Answer 1

I may be misinterpreting your question, but it's possible that you are making this much harder than necessary.

(Note: I had trouble dealing with the .zip file in R, so I just downloaded and unzipped it in the OS).

library(rgdal)
library(ggplot2)

setwd("< directory with shapefiles >")
map <- readOGR(dsn=".", layer="vg250_gem", p4s="+init=epsg:25832")
map <- spTransform(map, CRS("+proj=longlat +datum=WGS84"))

nPolys <- sapply(map@polygons, function(x)length(x@Polygons))
region <- map[which(nPolys==max(nPolys)),]
plot(region, col="lightgreen")

# using ggplot...
region.df <- fortify(region)
ggplot(region.df, aes(x=long,y=lat,group=group))+
  geom_polygon(fill="lightgreen")+
  geom_path(colour="grey50")+
  coord_fixed()

Note that ggplot does not deal with the holes properly: geom_path(...) works fine, but geom_polygon(...) fills the holes. I've had this problem before (see this question), and based on the lack of response it may be a bug in ggplot. Since you are not using geom_polygon(...), this does not affect you...

In your code above, you would replace the line:

ggmap(al1) + geom_path(data=as.data.frame(Poly.coords), aes(x=X1, y=X2))

with:

ggmap(al1) + geom_path(data=region.df, aes(x=long,y=lat,group=group))