GIS for Transportation: Principles, Data and Applications

5.1 Network Analysis


Network analysis can be used to solve many different transportation problems that would be very challenging to solve otherwise. A prerequisite to performing network analysis is that you have a network model. In ESRI’s terms, this is a network dataset. We walked through an exercise to construct a network dataset in the last lesson. Of course, it is not a requirement of network analysis that you construct your own network model. There are a number of commercially available network models you can use instead.

The types of problems which network analysis can be used to solve are quite varied. One common characteristic of the algorithms that power each is that they involve determining the cost of one or more routes through the network. The cost is most commonly based on time or distance, but you can define a cost attribute any way you want. For example, you might score each edge in the network based on its scenic value. You could then create a cost parameter based on the scenic score and use the solver to find the most scenic route.

ESRI provides 6 out-of-the-box network analyses as a part of Network Analyst. ESRI terms these network analyses “solvers.” The solvers are listed below along with a brief description of each:

Route Solver

The route solver determines the best route between two or more points. Most of us use this network analysis on a regular basis. Whenever you use Google Maps or a comparable service to get directions from one location to another the service is conducting a network analysis to determine the best, typically fastest, route. This solver can route any number of points according to a specified order (i.e., the traveling salesman problem) or the most efficient order.

Image on left: route solver- find the quickest route between two stops using a time-based cost attribute. Image on right: route solver - find the shortest parth between two stops using a distance-based cost attribute.
Figure 1 - Route Solver: Time vs. Distance Based Cost
Credit: ESRI

Closest Facility Solver

This solver is used to determine the closest facility to a given location. The term facility can be a bit misleading. For example, this solver could be used to determine the closest ambulance to an accident scene. In this case, using ESRI’s terminology, the ambulances would be considered facilities.

Closest facility solver: Find the routes from an incident to all facilities that can be reached within 15 minutes of driving.
Figure 2 - Closest Facility Solver
Credit: ESRI

Service Area Solver

The geographic region which can reach a designated facility in a certain period of time (or vice versa) is termed a service area. To determine the bounds of this area, you can use the Service Area Solver.

Service Area Solver: Find all roads that are within a 10-minute drive of a facility and then bound the roads by a polygon.
Figure 3 - Service Area Solver
Credit: ESRI

Origin-Destination (OD) Cost Matrix Solver

The OD cost matrix solver is generally used to determine the distances of the fastest routes between a set of origins and a set of destinations. Although the path between each origin and destination is often represented as a straight line, the route which corresponds to the time and distance costs between each pair of locations follows the street network.

OD cost matrix solver: Find the least-cost paths between each origin and its fours closest destinations and then store the values in an attribute table.
Figure 4 - OD Cost Matrix Solver
Credit: ESRI

Vehicle Routing Solver

The vehicle routing solver is typically used to determine the most efficient routes for a fleet of vehicles tasked with servicing a series of stops.

Vehicle routing problem solver: find the routes for a fleet of vehicles so that many orders are efficiently serviced and time windows, driver breaks, and vehicle capacities are honored.
Figure 5 - Vehicle Routing Solver
Credit: ESRI

Location – Allocation Solver

The location-allocation solver can be used to determine how effectively a facility site is servicing locations which have a need for its services. As such, it can be used to select the best location for a facility from a series of candidate locations.

location allocation solver
Figure 6 - Location-Allocation Solver
Credit: ESRI

Assignment 5-1 (15 points)

Watch this video from ESRI’s 2010 User Conference (71 minutes) which talks about conducting network analyses using the Network Analyst extension to ArcGIS. Address the following questions and submit your responses in the form of an M.S. Word document to Assignment 5-1 in Canvas.

  1. What is the difference between a Euclidean path and a network path? (3 points)
  2. What is a restriction barrier? Provide an example of when you might use one. (3 points)
  3. What is a scaled cost barrier? Provide an example of when you might use one. (3 points)
  4. What type of cost attribute would you generally use to find the best driving route? How about for a walking route? (3 points)
  5. The service area solver and the location-allocation solver are somewhat similar. Briefly describe what differentiates them. (3 points)

Assignment 5-2 (20 points)

In Assignment 4-2 you downloaded the ESRI ArcGIS Desktop Tutorial Data and completed Exercise 1 of ESRI’s ArcGIS Network Analyst Tutorial. For this assignment, you will complete Exercise 3 and Exercise 5. As you’re completing the exercises, address the following questions and submit your responses in the form of an M.S. Word document to Assignment 5-2 in Canvas.

  1. Exercise 3: Finding the best route using a network dataset.
    1. Will Network Analyst use historic traffic data if you do not specify a day and time for the route? (1 points)
    2. Include screen shots of your fastest route before and after you implemented a point barrier. (4 points)
    3. Add a polygon barrier somewhere along your route and re-solve. Include a screen shot of the resulting route. (2 points)
    4. Find the shortest route by changing the impedance in the analysis settings to distance (i.e., meters) in the analysis settings window and re-solving. Include screen shots of both the map of the new route and the analysis settings window. (2 points)
    5. Did Network Analyst use historic traffic data in determining the shortest route? (1 point)
  2. Exercise 5: Calculating service areas and creating an OD cost matrix.
    1. Include a screen shot of the service area map both before and after you move a facility and re-solve the analysis. (i.e., 2 screenshots)(4 points)
    2. What does ArcMap do if a store is inside multiple polygons and you join the stores features to the polygon features? (2 points)
    3. Include a screen shot of your final map showing both the results of the OD cost analysis and the service area analysis displayed on the same map. (4 points)