In a previous post, I outlined my workflow for preparing maps in R. Today I had to add a graticule, a grid of latitude and longitude lines, to my maps. That’s easy enough to do with unprojected maps, as the plot coordinates are latitude and longitude, so your X and Y axes are already graticules. But if you’ve projected your data, the plot coordinates are on a different scale, so you need to do a bit of tuning.
I couldn’t find a direct way to do this in the R
sp package. However,
sp for ‘spatial’) is slowly being replaced by
sf for simple
support graticules. Here are the steps required to add them to your plots:
We can use
raster::getData to get our map data again. It’s
straightforward to convert objects from
format and back, with the functions
st_as_sf (to convert from a
as (to convert from
sf* to a
object). As it turns out,
getData also supports downloading data directly
## Linking to GEOS 3.6.2, GDAL 2.2.3, PROJ 4.9.3
## Loading required package: sp
us <- getData("GADM", country = "USA", level = 1, path = "./data/maps/", type = "sf") canada <- getData("GADM", country = "CAN", level = 1, path = "./data/maps", type = "sf") mexico <- getData("GADM", country = "MEX", level = 1, path = "./data/maps", type = "sf")
This uses the undocumented type argument, set to
sf. Given that it’s not
documented, it may change in future, be warned!
You can also use the function
st_read to read shapefiles directly:
greatlakes <- st_read("data/maps/greatlakes.shp")
## Reading layer `greatlakes' from data source `/home/smithty/blogdown/content/tutorials/data/maps/greatlakes.shp' using driver `ESRI Shapefile' ## Simple feature collection with 2 features and 5 fields ## geometry type: POLYGON ## dimension: XY ## bbox: xmin: -365240.6 ymin: -2741892 xmax: 1888977 ymax: 509590.2 ## proj4string: +proj=laea +lat_0=45 +lon_0=-100 +x_0=0 +y_0=0 +a=6370997 +b=6370997 +units=m +no_defs
In the previous tutorial, I used
bind to combine two
sf we need
na <- rbind(us, canada, mexico)
Plotting complex vector maps like this can be a slow process, especially when you’re constantly tweaking and adjusting them. You can speed this up by simplifying the layers:
na.simp <- st_simplify(na, dTolerance = 0.01)
## Warning in st_simplify.sfc(st_geometry(x), preserveTopology, dTolerance): ## st_simplify does not correctly simplify longitude/latitude data, dTolerance ## needs to be in decimal degrees
On my laptop, plotting the original map takes a minute or more, compared to
2 seconds for the simplified vector. I set the tolerance by trial and
error. The higher the tolerance, the smoother the map will be. At 0.01, it
still looks nearly identical at the scale I’m plotting it, but is much
smaller and faster to plot.
sf does warn me about not correctly
simplifying the data, but since I’m only using this for display that’s not
a concern. I wouldn’t simplify a vector if I was going to use it in an analysis.
When it comes to plotting, we need to tell R to plot only the geometry. By default it will plot multiple maps, one for each attribute. That’s not what we want here.
plot(st_geometry(na.simp), xlim = c(-130, -70), ylim = c(35, 45))
To project our unprojected data, we need to define a projection, and transform the object.
laea = CRS("+proj=laea +lat_0=30 +lon_0=-95") na.la <- st_transform(na.simp, laea) plot(st_geometry(na.la), xlim = c(-500000, 2000000), ylim = c(-400000, 2100000))
We can add layers just as we did in the previous post:
gl.la <- st_transform(greatlakes, laea) plot(st_geometry(gl.la), col = 'lightblue', add = TRUE)
You can also mix
Spatial* objects on the same plot, as long as
they’re in the same projection.
Now we have everything we need to add graticules to our map. This includes the map we want to plot, and the CRS data for the graticules we want to overlay. In our case, we’ll use the original, unprojected layer as the source our CRS:
plot(st_geometry(na.la), xlim = c(-500000, 2000000), ylim = c(-400000, 2100000), graticule = st_crs(na.simp), bgc = 'lightblue', ## Background color for the ocean col = 'white', axes = TRUE) plot(st_geometry(gl.la), col = 'lightblue', add = TRUE)
If you want to specify the location of the graticules, you can use the
lon to specify where you want them.