A tessellation on a linear network L is a partition of the
network into non-overlapping pieces (tiles). Each tile consists of one
or more line segments which are subsets of the line segments making up
the network. A tile can consist of several disjoint pieces.
The data frame df should have columns named
seg, t0, t1 and tile.
Each row of the data frame specifies one sub-segment of the network
and allocates it to a particular tile.
The seg column specifies which line segment of the network
contains the sub-segment. Values of seg are integer indices
for the segments in as.psp(L).
The t0 and t1 columns specify the start and end points
of the sub-segment. They should be numeric values between 0 and 1
inclusive, where the values 0 and 1 representing the network vertices
that are joined by this network segment.
The tile column specifies which tile of the tessellation
includes this sub-segment. It will be coerced to a factor and its
levels will be the names of the tiles.
If df is missing or NULL, the result is a tessellation
with only one tile, consisting of the entire network L.