Netgraphs: a useful tool for planning ferry networks

A commonly used tool in European countries for planning periodic rail networks is the netgraph. Netgraphs provide a strategic visualisation of line connections and network capacity. They can also be a great aid for planning ferry networks with minor modifications to standard protocols to make them better suited to waterborne transport.

A netgraph is made up of three main elements:

• Nodes, which represent terminals and other interchanges in the network
• Edges, which connect the nodes and represent line segments in the network. The service frequency is depicted by the number of lines displayed (eg a double line means two services per hour).
• Time event labels, which represent the departure and arrival times of a transit unit at a node. By convention, the departure time is set apart from the node box and the arrival time is immediately adjacent.

Netgraphs only work for periodic, or strict clockface timetables. This is because a periodic timetable follows a repeating pattern. Events occurring in one hour are repeated in all hours. Periodic timetables are ideally suited to urban ferry systems where a repeating pattern in the timetable makes ferry to ferry and intermodal connections easier to plan. And where vessel movements are more regular and predictable, safety and operational efficiency performance also improves.

Figure 1 below shows a sub-section of a netgraph, based on a periodic timetable devised (but not adopted) for Sydney Ferries. It has some "bells and whistles" not included in regular netgraphs to make it more useful for a ferry system.

Fig. 1: Sub-section of a netgraph for a possible periodic Sydney Ferry timetable

The red circle with opposing arrows is an extra time event, showing when and where inbound and outbound vessels cross. Cockatoo Island is a dual berth stop and the letters S and N denote which side of the pontoon the ferries berth. In combination, these additions to netgraph protocol help avoid systemic berthing conflicts.

A netgraph also helps highlight where timed transfers are scheduled at a node. In this case, passengers can transfer between the red inner harbour line at Cockatoo Island and the yellow Parramatta River line with short, convenient waits in both directions. Note that the dotted red line represents a peak only service while the solid red line operates peak and off peak.

At a network wide level, all berthing assignments and ferry to ferry connections can be seen in a single schema as shown in the netgraph in Figure 2 below.

Figure 2: A possible periodic timetable for Sydney Ferries (off peak only)

Line colours signify vessel class, and highlight the potential for network modularity. By making line groupings almost entirely self contained - with a few exceptions, each berth is dedicated to one class of vessel only - it is possible for landing interfaces to be customised to the vessel. The advantage of this is that it is then possible to have faster passenger exchange at those berths and therefore fewer delays in the ferry system.

This is an abridged version of a paper presented at the Australasian Transport Research Forum conference in Canberra, 30 September 2019.