SpatialPolygonDataFrame to raster while keeping categorical values

Question

I have a SpatialPolygonDataFrame which I need to convert to raster in R while keeping the categorical values of the polygon. I have tried various approaches but I have always either lost the values or converted them to numerical, which is not what I want. Can anyone help?

Answer 1

Factors are stored as numbers, so a raster of categorical values is just a raster of numbers with a lookup-table for the labels (which is the same as a standard vector of factors in R).

To do what you want using modern R spatial classes, use sf and terra:

 library(sf)
 library(terra)

Read the shape:

 sh = st_read("./kasanka habitats.shp")

The "Coverage" variable (which I think is what you want to rasterize) is character at this point so we convert it to factor:

 sh$Coverage = as.factor(sh$Coverage)

create a basis for rasterizing based on the extent of the shape:

 y = rast(ext(sh),200,200)

now rasterize:

 tsh = rasterize(as(sh,"SpatVector"), y,field="Coverage")

and plot:

 plot(tsh)

Note this is still a raster of numbers, so if you try writing it to some file format you might lose the labels.

Answer 2

Here is Spacedman's solution using a more direct path

library(terra)
sh = vect("./kasanka habitats.shp")
y = rast(sh, 200, 200)
tsh = rasterize(sh, y, field="Coverage")