GIS Based Network Analysis for The Road Network

Introduction

Network analysis is a process to solve the practical problem by analysis in network model, such as path analysis, service area analysis, and find closest facility. At present, network analysis has been widely used in many industries. We will give you some examples, such as electronic navigation, telecommunications, transportation, tourism. As we all know, in the world of geographic information, public infrastructure (power facilities, telecommunications, cable TV network, road traffic, water network, etc.) is abstracted as a network system composed of many interconnected lines. As long as they have a network system that they need to manage and analyse in spatial, they can take advantage of this solution.

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

1. What is the shortest route from point A to point B
2. In the tourist attractions, how to choose rote which passing the most points of the interest at a specific time?
3. How big is the coverage area of the customers of a newly opened supermarket?
4. How to dispatch the recent fire engine for the rescue in the event of a fire?
5. How can a distributor deliver all his delivery tasks in the shortest time?

Technical Step

1. Build Network Model

The first step to achieve network analysis is create a network model dataset. The Network Analysis will not be able to execute If we run it for line dataset only, we have to create a network model dataset of it. There are two kinds of SuperMap network models: transportation network model and public facilities network model. Both models can be represented by two-dimensional network and three-dimensional network.

The main differences between the two network models are as follows:

1. Transportation network model is a non-directional network, and the most common one is route transportation network. Traffic network analysis is usually used for route search and location.
2. While, The facility network model is a directional network, usually including natural gas pipelines, rivers and so on. In this webinar, we mainly discuss the traffic network model.

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

As shown in the figure above, the network not only has the abstracted topological relationship between arc and nodes, also has geometric location feature and geographic attribute feature of GIS spatial data. What do we mean by topological relationship? Topological relationship is the cross-correlation between geographic object in spatial location, such as the connection relationship of nodes and lines, and lines and polygons.

Thefore, the following describes the basic conceps involved in the network model. Those are

1. nodes, which are the places that arcs connect network
2. Arcs, arc segment is an edge in the network connecting with the other arcs by nodes.
3. Network resistance, which is the cost of nodes and arcs that is basically stored in the attribute field as the resistance field
4. Center point
5. Obstacle edges and obstacle points
6. Turn table, which saves the cost of turning. Turning itself is the process that an arc passes through the middle node to arrive the adjacent arc

To create a network model, we need to pass the following preparation before build the network data

First, prepare the dataset of building the network data. Point dataset, line dataset, and network dataset can be used to build network data. In which, point data is optional, even if you do not select the point data, you still can build a network dataset.

Second, prepare the field information of the network dataset. You need to ensure that the line data to build the network contains a field that denotes the network resistance, such as the fields that denotes the time and distance information. This resistance can be used as the weight later on. If you want to do route analysis and you willing to generate the text information of the path table, please make sure that the line data to build the network contain the fields of the required instruction, such as road name or site name.

2. Optimal Path Analysis

Now we will perform optimal path analysis. The optimal path analysis is one of the commonly used function of network analysis. The optimal path analysis is used to find the path of the least resistance between two points in the network. The least resistance has different meaning: such as those based on a single factor like shortest time, the lowest cost, the best traffic condition, or others. The shortest route is a special case of the single factor of the optimal path.

The first figure is showing the difference result of optimal path analysis if we checked lest edges parameters. These parameters will analyse the route with the least number of edges. For example, the number of edges of the red route is less than the number of edges of the green route. But actually, the overall length of the resulting routes may not be the shortest.

Before performing any network analysis based on the network dataset, we need to set the environment settings. The environment settings window is used to set the global parameters of network analysis.

3. Traveling Salesman Problem (TSP)

Traveling Salesman Problem (TSP) Analysis is unordered path analysis. The TSP Analysis can The TSP analysis can decide the order to visit the nodes, and its target is to get the minimum (or close to the minimum) sum of the travel-route impedances.

The difference between TSP analysis and the optimal path analysis is the method to visit all the network nodes. The former can decide the visiting order of the nodes, but the latter must visit the nodes according to the specified order. The figure shows that the optimal path analysis will be from stop 1 to stop 2, while the TSP analysis will be from stop 1 to stop 4, because this is the shortest distance.

4. Closest Facility Analysis

The basic concept of closest facility analysis is to find the nearest facility point to the known point (event point). You can find one or more facility points that can be reached by the minimum cost. The analysis result is the optimal path between the event point to the facility point. For example, a traffic accident happens, the injured people need to be sent to the nearest hospital for emergency treatment within 10 minutes, the surrounding hospitals which cannot be reached within 10 minutes won't be taken into consideration. In this case, the location of the accident is an event point and the surrounding hospitals are facility points. In essence, the Closest Facility Query is path analysis as well. During the analysis, we should consider the sections that have barrier points and barrier edges that cannot be passed.

5. Service Area Analysis

Service area analysis is to conditionally find the scope of the service area that the specified service stations can provide. First you need to understand two basic concepts:

1. Service stops: The center point of the service, providing the location of the service, such as a supermarket, post office, community hospital, etc
2. Service area: The area that within a certain impedance taking specified point as the center and containing all the accessible edges. Simply, it refers to the area that is served by a location according to some certain conditions to provide a particular service, e.g., the governed area of a community police station according to the administrative divisions.

In the network analysis, the network nodes with resources are abstracted as service stops, the maximum distance of the service stops is abstracted as service radius. In the analysis results you can get the service network and service area (polygon). The analysis is generally used to evaluate and analyse the service scopes of the public facilities, in term of route, such as post office, hospital, supermarket, etc., so as to provide the references for selecting the optimal locations of the public facilities.

6. Location-Allocation Analysis

Location-Allocation is to select the optimal location for one or more service providers in a certain region, so that the facilities can be one of the most cost-effective ways to provide services and merchandises. Location-Allocation is more than just how to locate, which also needs to allocate the demands of the demanding points to the appropriate facilities service area.

For example: A city has 15 hospitals whose distributions are showed in the figure below. Now you want to select 7 from the 15 hospitals as the city's centralized hospitals for the physical examination of the college entrance test. It needs to cover the areas as many as possible to facilitate the physical examination of the candidates. You will select 7 optimal locations from the 15 hospitals for the physical examination (as shown in the left figure, the red symbol represents the 15 hospitals). The hospital of the physical examination must ensure that it takes the candidates less than 30 minutes to walk or ride. Based on this condition, the Location-allocation analysis will give the best sites and circle the service range of each hospital (As shown in the right figure, the points denoted in different colours represents the 7 optimal locations for the physical examination).

Benefits we can take

SuperMap provides various traffic analysis which escalate the location insight based on network routing and analysis. As one of the main components in a base map, road that preserved in a line dataset is a powerful object to give you an intangible solution, it will be turned into tangible once you successfully analyse them further. Furthermore, both the process and result are available to perform in various terminal such as Desktop (iDesktop), Mobile Application (iMovile and iTablet), and Web Based as well (iServer and iClient). 

Should you want to watch the tutorial of this article, please visit our video by this link and download the material here.

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