Messed up polygons shape when combining google map with Spatial Polygons

Question

I´m having problems when combining a map from Google Map Api, with a map constructed with geom_polygon from ggplot2. When I plot each map on their own, nothing weird shows up, but when I do combine them, some lines(straight) appear, messing up the boundaries I intend to highlight.

1) The data I´m using to construct the polygos with the black borders comes from this link. The exact url of the file is in my code below. Sometimes it is necessary to unzip the file manually, therefore the code below for downloading the data may not work:

path <- getwd()
fileName <- "R05.zip"
if (!file.exists(fileName)) {
    urlFile = "http://www.censo2017.cl/wp-content/uploads/2016/12/R05.zip" 
            download(urlFile, dest = "./R05.zip", mode ="wb")
}
if (!dir.exists("./R05")) {
    unzip("R05.zip")
}

2) Then I load the shapefile with which I´ll construct the polygons.

distritos <- readOGR( dsn= paste(getwd(), "/R05", sep = ""),
                  layer="Distritos_Censales", 
                  encoding = "UTF-8", stringsAsFactors = FALSE)

3) And select the administrative division I´m interested in

 distritos <- distritos[distritos@data$DESC_COMUN=="QUILPUÉ", ]

4) And then the districts (polygons) I´m interested in:

distritos <- distritos[distritos@data$DESC_DISTR=="EL RETIRO" |
                           distritos@data$DESC_DISTR=="BELLOTO NORTE" |
                           distritos@data$DESC_DISTR=="VALENCIA" | 
                           distritos@data$DESC_DISTR=="MENA" |
                           distritos@data$DESC_DISTR=="BELLOTO SUR" |
                           distritos@data$DESC_DISTR=="ALTO QUILPUÉ" |
                           distritos@data$DESC_DISTR=="EL SAUCE", ]

5) Construct the base map from the boundaries of distritos. For that, I use the function to get the center of a map from this StackOverflow question

bbox(distritos)
MapCenter <- function(x1, y1, x2, y2){
    center.x <- x1 + ((x2 - x1) / 2)
    center.y <- y1 + ((y2 - y1) / 2)
    center <- c(center.x, center.y)
    center
}
mcdistritos <- MapCenter(bbox(distritos)[1,1], bbox(distritos)[2,1], 
bbox(distritos)[1,2], bbox(distritos)[2,2])
basemap <- get_googlemap(mcdistritos, zoom = 13, 
                          maptype = "roadmap",
                          color = "bw",
                          style = "feature:administrative|element:labels|visibility:off")
basemap <- ggmap(basemap , extent = "device")

6) prepare the shapefile data to plot it with ggplot2

distritos.fort <- fortify(distritos, region = "DESC_DISTR")

7) Plot together both maps

basemap + 
geom_polygon(data = distritos.fort, 
                    aes(x = long, y = lat),
                        colour = "black", 
                          fill = NA) + coord_map()

I tried by zooming out the base map, in case the polygon boundaries were messed up cause they didn´t fit in the base map, but I got the same result, just a smaller map. Does anyone know how to fix it?

Just a suggestion, you can replace distritos@data$DESC_DISTR=="EL RETIRO" | distritos@data$DESC_DISTR=="BELLOTO NORTE" | distritos@data$DESC_DISTR=="VALENCIA" | distritos@data$DESC_DISTR=="MENA" |... with something much shorter, like disitros@data$DESC_DISTR %in% c("EL RETIRO", "BELLOTO NORTE", "VALENCIA", "MENA", ...) — Gregor Thomas

Jeremy Voisey Jeremy Voisey · Accepted Answer · 2017-05-24T02:14:40

You need to add a group mapping to your aesthetic. e.g.

geom_polygon(data = distritos.fort, 
                    aes(x = long, y = lat, group = group),
                        colour = "black",
                          fill = NA) + coord_map()

At the moment you've got one continuous path. The group aesthetic separates your data into different polygons.

I wasn't able to extract your data, so I don't know the exact mapping you require for group. But looking at the other fortify examples & documentation, I believe it is group = group

Messed up polygons shape when combining google map with Spatial Polygons

1 Answers