8

I'm using ggmap to grab Washington DC and I have a shapefile that I downloaded from here:

library(ggplot2)
library(ggmap)
library(maptools)

dc = get_map(location = 'DC', zoom = 12)

nhbds = readShapePoly("Census_Tracts_-_2010.shp")

ggmap(dc) + 
  geom_polygon(aes(x = long, y = lat, group = group), 
           data = nhbds,
           alpha = 0.8, 
           color = "black",
           size = 0.2)

Here's the plot

I'd like to clip the map to the borders of the city only, getting rid of Virginia and Maryland. And I don't actually want to overlay the tracts.

Any simple ways to do this? Here's a nice map that looks like what ultimately I want to accomplish:

DC map

1 Answer 1

12

Robin Lovelace has provided a nice little function to download a ggmap object and convert it to a raster. Using this you could do:

library(ggmap)
library(raster)
library(rgdal)

# courtesy R Lovelace
ggmap_rast <- function(map){
  map_bbox <- attr(map, 'bb') 
  .extent <- extent(as.numeric(map_bbox[c(2,4,1,3)]))
  my_map <- raster(.extent, nrow= nrow(map), ncol = ncol(map))
  rgb_cols <- setNames(as.data.frame(t(col2rgb(map))), c('red','green','blue'))
  red <- my_map
  values(red) <- rgb_cols[['red']]
  green <- my_map
  values(green) <- rgb_cols[['green']]
  blue <- my_map
  values(blue) <- rgb_cols[['blue']]
  stack(red,green,blue)
}

dc <- get_map(location = 'DC', zoom = 12) 
dc.rast <- ggmap_rast(map = dc) # convert google map to raster object
nhbds <- readOGR("Census_Tracts_-_2010", "Census_Tracts_-_2010") # use rgdal to preserve projection
dc.only <- mask(dc.rast, nhbds) # clip to bounds of census tracts

# prep raster as a data frame for printing with ggplot
dc.df <- data.frame(rasterToPoints(dc.only))
ggplot(dc.df) + 
  geom_point(aes(x=x, y=y, col=rgb(layer.1/255, layer.2/255, layer.3/255))) + 
  scale_color_identity()

enter image description here

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.