Changing the Plot Projection in ggplot · Data Science with R for the Life Sciences

We’ve talked about how different spatial data have different projections which have different units
But you may have noticed that something a little funny happens with these units when we plot data using ggplot
Let’s start where we did at the beginning of the last lesson
Having already loaded the packages we need and imported the Harvard Forest soils data and digital terrain model

library(sf)
library(stars)
library(ggplot2)

harv_soils <- st_read("data/HARV/harv_soils.shp")
harv_dtm <- read_stars("data/HARV/HARV_dtmFull.tif")

Let’s look at the coordinate reference systems for these two data sets

st_crs(harv_soils)
st_crs(harv_dtm)

So both data sets in are UTM Zone 18 North
If we plot the raster data on it’s own it is in UTM units

ggplot() +
  geom_stars(data = harv_dtm)

It has the large numbers we’ve seen before on the axes
Now let’s add the soils data

ggplot() +
  geom_stars(data = harv_dtm) +
  geom_sf(data = harv_soils, alpha = 0)

Now all of a sudden we have latitudes and longitudes instead of our UTM coordinates
This happens because by default geom_sf changes the units to latitude and longitude values
To change the units to another projection, like the projection of the data, we use coord_sf
Which takes a CRS and shows the axes in those units
So if we want this graph in the units of the projection of our data we can look up the CRS and use that

ggplot() +
  geom_stars(data = harv_dtm) +
  geom_sf(data = harv_soils, alpha = 0) +
  coord_sf(datum = st_crs(harv_dtm))

We could also use the numeric code for the CRS instead if we want to