Setting up a Network Allocation Model
Network analysis involves calculating the least-cost path along the network. Cost is also referred to as an impedance. Cost is usually measured in terms of distance, time, or money. For example, if time is the most relevant cost, the least-cost path from a fire station to a fire will be the quickest path, not the shortest one. In this step, you will use the location-allocation solver in Network Analyst to create a network allocation model. A network allocation model helps you choose which facilities from a set of facilities to choose from based on their potential interaction with demand points. In this scenario, the facilities will be the proposed fire station locations. The demand points will be the history of incidents within the Arcata Fire Protection District. The impedance is the travel time from facilities to demand points. This impedance will constrain the model to specific time windows that you define. The objective will be to chose three facilities among the candidates, maximizing the number of demand points that could be reached within a set amount of time. In the Network Analyst Toolbar, choose New Location_Allocation.
In the table of contents, a new group layer is added. This group layer is comprised of empty memory feature classes. These data types behave similar to shapefiles but only exist temporarily in the computer’s memory. When you close ArcMap, you will no longer have access to these layers. They must be exported if you want to make them permanent. However, they are temporary for a reason. By using memory feature classes, you can re-run our model multiple times with different sets of parameters, without unnecessarily creating shapefiles that you may not need to keep in the long term. This saves on overall disk storage space and makes managing data easier as you conduct your analysis. In the Network Analyst window, an empty network allocation model is added. It is here that you will enter your model parameters. Each time you adjust your model parameters, you will do so here as well.
Note: It is possible to create multiple network allocation models in one map document. Each represents a different model. In this exercise, you will only work with one. Be careful not to inadvertently create multiple models.