An object of class "ssf" represents a real-valued or
vector-valued function that has been evaluated or sampled at an
irregular set of points. An example would be a spatial covariate
that has only been measured at certain locations.
An object of this class also inherits the class "ppp",
and is essentially the same as a marked point pattern, except
for the class membership which enables it to be
handled in a different way.
There are methods for plot, print etc; see
plot.ssf and methods.ssf.
Use unmark to extract only the point
locations, and marks.ssf to extract only the function values.