Demand-Based Site Selection for Fire Stations Using a Network Allocation Model

Adjusting the Analysis Setting of the Model

The network allocation model also has global properties that must be set. So far you have been working with attributes for individual model parameters such as facilities and demand points. Here you will adjust the analysis settings of the entire network allocation model. These settings are global, meaning they will influence all of the model parameters.

an image of the Properties button in the Network Analyst Window

Click on the Location-Allocation Properties button in the Network Analyst Window. 

Navigate to the Analysis Settings tab. Verify that the Impedance is set to Minutes (Minutes). There are two choices for the Impedance property, time and distance. The Impedance property specifies the cost for traveling along the edges of the network. By setting the property to minutes, the model will define the least cost path as the quickest path, not the shortest one. When calculating a least-cost path along the network, the direction of travel can affect distance and travel time. In this scenario, travel will always originate from the fire stations to the location of the incident. Verify that the Travel From setting is Facility to Demand. Leave all other settings in the Analysis Settings tab as default. Then navigate to the Advanced Settings tab.

An image of the Location-Allocation Analysis Settings Tab

In the Advanced Settings tab, there are several problem types to choose from. The Problem Type settings determine how the model will calculate allocated weight to the chosen facilities. The methods used largely depend on the needs of problem you are trying to solve. For example, a common Problem Type for emergency responders is Maximize Coverage. The premise is that emergency services are required to arrive at all demand points within a specified response time. This method is a straightforward calculation of allocated demand. Any demand points that fall outside of the impedance cutoff, the response time window, are not considered in the calculation. All of the demand points that fall within the time window cutoff are counted. In addition, the full weight of the demand point is used in the calculation. If two or more facilities are competing with one another, the weight from a demand point only goes to the nearest facility. The demand weight is summed up for each candidate facility to determine the allocated demand. The facility with the highest allocated demand is chosen.

Verify that the Problem Type is set to Maximize Coverage. The Facilities to Choose setting specifies the total number of fire stations that can be chosen by the model. In this scenario, the Arcata Fire Protection district is interested in choosing only three sites from the list of proposed locations. Verify that the Facilities to Choose is set to 3.

The Impedance Cutoff setting defines the boundaries of a service area originating from each candidate facility measured by the least-cost path along the network. Since the Impedance setting in the Analysis Settings tab was set to minutes, the number entered here will define a service area based on a response time window.  Set the Impedance Cutoff to 2. Any path to a demand point with a higher cost than two minutes will not be considered. Leave all other settings as default and click OK. With these settings, the model will choose only three of the potential sites. Travel will be calculated from each of the potential sites to the demand points. However, any demand points greater that two minutes away from a proposed location will not be counted.

An image of the advanced settings tab