levels() needs an update

[PennMGIS]

In lesson: Introduction to Geospatial Raster and Vector Data with R
Episode7: Explore and Plot by Vector Layer Attributes
https://datacarpentry.org/r-raster-vector-geospatial/07-vector-shapefile-attributes-in-r/index.html

levels(lines_HARV$TYPE) should return the unique values within TYPE column containing categorical variables. However, in the exercise this actually returns NULL.

I did a little investigation and for this to work, when reading the spatial data you need to include "stringsAsFactors = T" parameter in str_read.

lines_HARV <- st_read("data/NEON-DS-Site-Layout-Files/HARV/HARV_roads.shp", stringsAsFactors = T)

Or even better, if st_read is not flagged with stringAsFactors =T, use...
levels(as.factor(lines_HARV$TYPE))

The reason for this is due to a change of R code in the way it handles string values & factors. A good explanation for this can be found at:
https://simplystatistics.org/posts/2015-07-24-stringsasfactors-an-unauthorized-biography/