For details of the point datasets, see raintrees and raintrees2. This accompanying dataset
gives information about the elevation in the study region.
It is a list containing two pixel images, elev (elevation in metres)
and grad (norm of elevation gradient). These pixel images are objects of class im.
Covariate values are continuous. Once discretized as wished, they can turn into categorical datasets
for the computation of all entropy measures. Moreover, they can be used
to build sensible sub-areas for Batty's and Karlstrom and Ceccato's entropies
(see the examples).