This function determines which tile of the tessellation Z
contains each of the spatial points
with coordinates (x[i],y[i]).
The result is a factor, of the same length as x and y,
indicating which tile contains each point. The levels of the factor
are the names of the tiles of Z.
Values are NA if the corresponding point lies outside the tessellation.
See Also
cut.ppp and split.ppp to
divide up the points of a point pattern according to
a tessellation.
as.function.tess to create a function whose
value is the tile index.