\BeginKnitrBlock{rmdnote}<div class="rmdnote">`raster` objects can contain values of class `numeric`, `integer`, `logical` or `factor`, but not `character`.
To use character values, they must first be converted into an appropriate class, for example using the function `factor()`. 
The `levels` argument was used in the preceding code chunk to create an ordered factor:
clay < silt < sand in terms of grain size.
See the Data structures chapter of @wickham_advanced_2014 for further details on classes.</div>\EndKnitrBlock{rmdnote}