Changing the Plot Projection in ggplot

• 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 <- read_sf("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

Do Task 6 of Harvard Forest Soils Analysis.