Directory under which all the coverages and a directory called 'info' are.
coverage
The name of the coverage we want to work with.
filename
The name of the file in the coverage directory that
stores the data. By default, it is called 'arc.dat'.
Value
This function returns a list with two elements. The first one is a data
frame containing the next fields (by columns):
ArcID
A number that identifies this arc.
ArcUserID
Identifier defined by the user.
FromNode
The node where the arc begins.
ToNode
The node where the arc finishes.
LeftPoly
The number of the polygon that is to the left of the arc.
RightPoly
The number of the polygon that is to the right of the arc.
NVertices
The number of vertices the arc has.
The second element is a list that stores the vertices of the arc. So, each element
in this list is also a list of two arrays: the first for the X coordinates
and the secod for the Y coordinates.