Facility and Transportation Network Analysis Overview

1. Facility Network Analysis

In our real world, the common facilities are municipal water network, transmission lines, gas pipelines, telecommunications networks, water system, etc. All the networks' resources are flowing directionally, which can be modeled and analyzed with the facility network.

The facility network analysis is one of the common functions of network analysis. It mainly processes a variety of connectivity analyses and tracing analyses.

The facility network analysis must follow some rules to build the network models. It must have the flow direction, which is the flow direction of the substance. The network direction depends on the network topology, and the locations of the source and sink.

Basics:

A facility network is composed of a group of edges and junctions. It can be used to express and model the real-world network facilities according to the specified connectivity rules. Users can specify the meaning and rules of the basic elements (points/lines) that compose the facility network to specify how the resources flow in the facility network.

Figure 1. Facility Network

Figure 1 is a simple network diagram. The thick blue line is the main pipeline composed of segments 1, 2, 3; the thin black segments 4 and 5 are the branch pipeline; the red junctions Node1 and Node 2 are the connected points of the main pipeline and branch pipeline.

• Edges: The established facility network generally includes the line objects, which are used to denote the flowing resource pipelines such as water pipelines, electric wires, natural gas pipelines, etc.
• Simple Edges: The edge that can only connect two junctions with its two endpoints, such as the branch pipelines 4 and 5 in the figure 1. The simple edges don't have the internal connectivity. If you need to add a new junction on it, then it must be split into two simple edges.
• Complex Edges: Besides the two junctions at the end, the complex edge also can be added junctions in its middle. As shown in the figure 1, the blue main pipeline connects the two branch pipelines at the junctions Node1 and Node 2, but it is not really be split, i.e., you can add as many junctions as you can.
• Note: Universal GIS Core (UGC) currently doesn't have the complex edges, i.e., all the complex edges will be split physically into simple edges.
• Junctions: The facility network generally includes the point objects, which denotes the junctions of two or more flowing resource pipeline, such as the pumping stations and valves of the water network, electric gates of the power grid, and supply points of the natural gas network.
• Source: The junctions that the resources flow out of, such as the power station and water station in the real world.
• Sink: The junctions that the resources flow into, such as the access points of the power grid and water network in the real world.
• Network Weight: Every network can associate with a group of weights. For example, there is a hydraulic weight in the water network, which associates with the edge's length. It represents that the water pressure continues to reduce with the water flowing due to the friction of the pipeline. A type of network weight can associate with one or more than one types of objects. The weight value can be 0 like all the values of the orphaned junctions that do not associate with any fields are 0.
• Valid Feature and Invalid Feature: All the edges and junctions of the facility network can be invalid due to some reasons (for example, the closed valve results in a segment of water pipeline that is obstructed). Invalid edges/junctions turns into network barriers. Valid or invalid feature can be denoted by a field.

The meanings of the numbers in the direction field are described in the following table:

Figure 2 below shows a river network after its direction is built. Different colors and lines denote distinct directions. Refers to the above table about the meanings of the numbers in the legend.

Figure2. Flow Direction Overview

2. Transportation Network Analysis

Overview

Network analysis facilitates the commerce and public services, as well as our daily life. The analysis results can provide an effective implementation proposal to help the user to make a more rational decision. Network analysis can help to solve the following practical problems:

• What is the shortest route from point A to point B?
• In the tourist attractions, how to choose a route which can past the most points of the interests at a time?
• How big is the coverage area of the customers of a newly opened supermarket? And how to determine its purchase volume?
• How to dispatch the recent fire engines for the rescue in the event of a fire?
• How can a distributor deliver all his delivery tasks in the shortest time?

In the world of geographic information, public infrastructure (power facilities, telecommunications and cable TV networks, road transportation, water network, etc.) are abstracted as network system composed by many interconnected lines. And the network model is the abstraction of the network system of the real world. Take the urban transportation network for example, the road and other linear features are abstracted as line segments, which are also called the network arcs. While the crossroads, the bus stops and the other point-like features are abstracted as points, in the network which are also called the network nodes. In the network model, resource information can transfer from a node to another node along the arc. We can consider the network is composed of the edges/lines, the intersected points/nodes and the other elements; it represents a possible path from one location to another one.

Basics:

The network is a model which is composed of a group of interrelated arcs, nodes and their attributes. The network can express the real world's roads, pipelines, etc.

Figure 3. Network Diagram

As shown in the Figure 3 above, the network not only has the abstracted topological relationship between arcs and nodes, also has geometric location feature and geographic attribute feature of GIS spatial data (topological relationship is the cross-correlation between geographic objects in spatial location, such as the connection relationship of nodes and lines, and lines and polygons).

The following describes the basic concepts involved in the network model:

• Nodes

Nodes are the places that arcs connects network, as shown above. Nodes can represent the real road intersections, rivers intersections, etc. The nodes and arcs respectively correspond to an attribute table, their adjacent relationships associate by the fields of attribute tables.

• Arcs

The arc segment is an edge in the network connecting with the other arcs by nodes. The arcs can represent the real-world highways and railways of the transportation network, the transmission line in the power network, the rivers of hydrological network, etc. The interconnected relationships between arcs have topological structure.

• Network Resistance

In our life, from the starting point, after a series of roads and junctions, we arrive the destination, which is bound to produce a cost measuring by distance, time and currency. In the network model, the cost of nodes and arcs is abstracted as the network resistance, and the information is stored in the attribute fields called the resistance field.

• Center Point

The center points are the discrete devices located at the nodes of the network and have the capabilities to accept or provide resources. Facilities are the substances, resources, information, management, cultural environment required by GIS, etc. For example, the school has educational resources and the students have to go to school to learn; the retail warehousing point stored the goods required by the retail outlets, and needs to deliver the goods to the various outlets. The center point essentially is a node in the network.

• Obstacle Edges and Obstacle Points

The problem of traffic jams in the city can be seen everywhere, which is a random and dynamic process, and it is no rules to follow. In order to reflect the real-time situation of transportation network, the traffic-jam arcs need to have the feature of temporarily inhibiting the passages of vehicles. When the traffic is back to normal, the attribute of the arc also can be set to normal in a real time. The concepts of obstacle edges and obstacle points can solve the above problems very well. The benefit to introduce the concepts of obstacle edges and obstacle points is that the obstacle settings are relatively independent, i.e., whether to set the obstacles has nothing to do with the current network environmental parameters.

• Turn Table

Turning is the process that a arc passes through the middle node to arrive the adjacent arc. Turning cost is the cost to complete a turning. Turn table saves the cost of the turning. The turn table must list all the possible turns of every crossroad, generally including the start arc field (FromEdgeID), the end arc field (ToEdgeID), the node identification field (NodeID) and the turn cost field (TurnCost). These fields correlate with the fields in arcs and nodes. Each record in the table dictates a arc cost passing the road node. Turn cost is usually directional, the negative cost values of the turning generally means to prohibit the turning.

For example, for the network analysis of the roads, we often encounter the crossroads, divergence, etc. The figure on the right is a crossroad diagram, and the table on the left is a turn table corresponding to the crossroad. In the turn table, there are the cost records that the vehicles turn at the crossroads.

Figure 4. Turn Table Diagram

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