Mapping in R using the ggplot2 package

One of my favorite packages for creating maps in R is ggplot2. No matter what, though, creating maps in R is trickier than doing it in a GIS system, particularly when you don't have 'on the fly' projection as you have in both ArcGIS and QGIS. To help you create maps on your own we share a typical use case below (including a coordinate system mismatch). In this example we use points and polygons by themselves but if you'd like to include tilemaps from Google, Stamen and others you should check out the ggmap package.

1. Read in the point and polygon data

Start by reading in the data. Our point data is in a comma-separated file with latitude and longitude values. Our polygons are a shapefile of NYC counties (also known as boroughs). You can get a similar file from the NYC Department of City Planning. Note that we're using the readOGR function from the package rgdal instead of readShapePoly from maptools. You'll see why later in the post.

  library(rgdal)
  library(ggplot2)
    # read in point data (tabular data)
  mapdata<-read.csv("mapdata.csv", stringsAsFactors=FALSE)
  head(mapdata)
##       id buildarea latitude longitude
## 1  10023    12.820    40.76    -73.99
## 2  10027    22.092    40.76    -73.99
## 3 10030A    26.081    40.76    -73.98
## 4 10030B    25.597    40.75    -73.98
## 5  10072     3.775    40.76    -73.82
## 6  10225    16.416    40.76    -73.97

  # the shapefile of NYC counties/boroughs. Careful about how you define the path
  # and layer. I always find this odd.
  counties<-readOGR("nybb.shp", layer="nybb")
## OGR data source with driver: ESRI Shapefile 
## Source: "nybb.shp", layer: "nybb"
## with 5 features and 6 fields
## Feature type: wkbMultiPolygon with 2 dimensions

2. Make some simple maps using ggplot()

Now we can create the maps in the same way we make non-geographic charts in ggplot. (If you know NYC, you know that the map is distorted — don’t worry we will fix this in the last step).

  # map the counties
  ggplot() +  geom_polygon(data=counties, aes(x=long, y=lat, group=group))

How about the points

  # map the points
  ggplot() +  geom_point(data=mapdata, aes(x=longitude, y=latitude), color="red")

Great, this is easy. Let's combine the two:

  # map both polys and points together
  ggplot()+geom_polygon(data=counties, aes(x=long, y=lat, group=group)) +  
  geom_point(data=mapdata, aes(x=longitude, y=latitude), color="red")

Shucks! Usually when you see something odd like this it's related to inconsisitent projections.

3. Make the projections consistent

Projecting geographic data is a pain no matter how long you've been doing it. Even though I've been mapping with R for many years I still need to refer to my cheat sheet with some common projections and examples of the code needed to project.

[Note that ggplot2 does have a function called coord_map() that can be used as a sort of on-the-fly projection but the function is still experimental so we’re sticking with this approach for now.]

Before projecting we need to know the existing projection/coordinate system for the layers. For the counties we can use the function proj4string. Note, though, that if you read in the shapefile using readShapePoly from the library maptools instead of readOGR the projection information will not be included.

proj4string(counties)
## [1] "+proj=lcc +lat_1=40.66666666666666 +lat_2=41.03333333333333 +lat_0=40.16666666666666 +lon_0=-74 +x_0=300000 +y_0=0 +datum=NAD83 +units=us-ft +no_defs +ellps=GRS80 +towgs84=0,0,0"

Well, the proj4 format is about as clear as mud. To get a more readable format you can look at the .prj file that came with shapefile. For this one you would see the following (note that what you’ll actually see is plain text — the nice formatting you see here comes from spatialreference.org):

prj

This is a good, common projection for New York City – Long Island State Plane (NAD83). But looking at the tabular point data file I see latitude and longitude coordinates (not X/Y) so I know that the point data is not projected. I need to project the point data to State Plane to match the polygon data. To do this we use the spTransform() function (from the rgdal package). But in order to use it we need to convert the points data frame to class SpatialPointsDataFrame.

class(mapdata)
## [1] "data.frame"
coordinates(mapdata)<-~longitude+latitude
class(mapdata)
## [1] "SpatialPointsDataFrame"
## attr(,"package")
## [1] "sp"

# does it have a projection/coordinate system assigned?
proj4string(mapdata) # nope
## [1] NA

# we know that the coordinate system is NAD83 so we can manually
# tell R what the coordinate system is
proj4string(mapdata)<-CRS("+proj=longlat +datum=NAD83")

# now we can use the spTransform function to project. We will project
# the mapdata and for coordinate reference system (CRS) we will
# assign the projection from counties

mapdata<-spTransform(mapdata, CRS(proj4string(counties)))

# double check that they match
identical(proj4string(mapdata),proj4string(counties))
## [1] TRUE

4. Back to the maps


# ggplot can't deal with a SpatialPointsDataFrame so we can convert back to a data.frame
mapdata<-data.frame(mapdata)

# we're not dealing with lat/long but with x/y
# this is not necessary but for clarity change variable names
names(mapdata)[names(mapdata)=="longitude"]<-"x"
names(mapdata)[names(mapdata)=="latitude"]<-"y"

# now create the map
ggplot() +geom_polygon(data=counties, aes(x=long, y=lat, group=group))+  geom_point(data=mapdata, aes(x=x, y=y), color="red")

Ahh, that’s better. Still not pretty, but we have the pieces working.

5. Final touches

Here we're going make the map a little nicer. Get rid of axes, new color gradient, clean up title etc.

ggplot() +  
    geom_polygon(data=counties, aes(x=long, y=lat, group=group), fill="grey40", 
        colour="grey90", alpha=1)+
    labs(x="", y="", title="Building Area Within 1000m")+ #labels
    theme(axis.ticks.y = element_blank(),axis.text.y = element_blank(), # get rid of x ticks/text
          axis.ticks.x = element_blank(),axis.text.x = element_blank(), # get rid of y ticks/text
          plot.title = element_text(lineheight=.8, face="bold", vjust=1))+ # make title bold and add space
    geom_point(aes(x=x, y=y, color=buildarea), data=mapdata, alpha=1, size=3, color="grey20")+# to get outline
    geom_point(aes(x=x, y=y, color=buildarea), data=mapdata, alpha=1, size=2)+
    scale_colour_gradientn("Building\narea (sq-km)", 
        colours=c( "#f9f3c2","#660000"))+ # change color scale
    coord_equal(ratio=1) # square plot to avoid the distortion

map

19 responses

  1. Just wanted to say thank you for a wonderful write up. Very clear and understandable for someone who is new to spatial data in R.

  2. This is great! I’d been struggling with a conversion of a GeoTiff file to a lon lat coordinate system to overplot with data and ultimately I gave up; the bar is high on this stuff, especially to non-geographers. It seems that your walkthrough will guide me through exactly what I need to do. Your writeup is super-clear and understandable and contains a real-life example — just what some of the R documentation is badly in need of. Well done & thanks.

    • Thanks! Did you end up solving your issue? The `raster` package is really great for this — `projectRaster()` might be what you need. But if possible you’d likely want to make a map with a projected (rather than geographic) coordinate system. Good luck.

  3. Great post.. much appreciated. Is there a way to only plot Manhattan and Brooklyn as opposed to plotting all five (5) boroughs.

    • Sure. You’d filter the county polygons to Manhattan and use this in place of polygons. Then you could use the over function from sp to identify the points that land in Manhattan and re-plot.

  4. Hi, I found your post by googling how to translate geographic coordinates to NYC borough names. The thing is, I have a big dataset of points in NYC and I want to assign each coordinate to his correspondent borough. I just want a new dataset with an extra column with the borough name, is this possible by using the packages that you used in this post? I already have a shapefile from NYC Boroughs. Thanks and great post!

  5. Hello,

    I am getting an error when I try to remove ticks and text from my ggplot axis… It says “Error: Don’t know how to add RHS to a theme object” –> is there a special packages to use theme?

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