Mapping polygon data based on properties

A common type of vector data is data with multiple polygons that each have associated properties
For Harvard Forest we have a map of soil types at the site
Let’s load it and map it using the sf package

library(sf)

harv_soils <- st_read("data/HARV/harv_soils.shp")

ggplot() +
  geom_sf(data = harv_soils)

We can see that we get a map with multiple polygons
If we look at the harv_soils object we’ll see that it’s a simple feature object with the geometry being POLYGON

harv_soils

Each feature is a single polygon with 5 associated fields: soil type, notes, two different soil type descriptions, and how well drained the soil is
We can include this information on our maps in the same ways we include it in other ggplot maps and graphs
Let’s start by coloring each polygon by it’s soil type
We do this by setting the mapping to have fill = TYPE_

ggplot() +
  geom_sf(data = harv_soils, mapping = aes(fill = TYPE_))

We can switch the color scheme to viridis like we did for raster data, but using the _d for discrete

ggplot() +
  geom_sf(data = harv_soils, mapping = aes(fill = TYPE_)) +
  scale_fill_viridis_d()

We can also plot different soil types on different subplots to help us focus on where individual soil types occur
We do this using facet_wrap just like we did for plotting tabular data

ggplot() +
  geom_sf(data = harv_soils) +
  facet_wrap(~TYPE_)

Notes