Scenario Linking

The scenario databases are linked to each other and, eventually, to a baseyear database via a tree structure: each scenario can refer to exactly one parent database; that parent can be either another scenario or a baseyear database. The baseyear database is the root of the tree. In this way, multiple scenarios may share the same baseyear.

When UrbanSim looks for a particular input database table, it traverses this chain looking for that table. The first table matching that name is used. In this way, any tables contained in the scenario database ``shadow'' or ``hide'' the same-named tables in the scenario's parent database(s).

Consider these example of how to create derivative scenarios:

• Scenario 1 is the base year plus a larger urban growth boundary, thus scenario 1's parent is the base year database. The UrbanSim scenario file will specify the scenario 1 database as the ``scenario-data''.
• Scenario 2 is the base year plus a major employer leaving the municipality. Scenario 2's parent is also the base year database. The UrbanSim scenario file will specify the scenario 2 database as the ``scenario-data''.
• Scenario 3 is the same as scenario 1, but with additional changes in the zoning laws to compensate for the larger UGB. Scenario 3's parent is scenario 1 and the UrbanSim scenario file will specify the scenario 3 database as the ``scenario-data''.
• Scenario 4 is the same as scenario 1, but with changes in the population demographics as a result of the larger UGB. Scenario 4's parent is scenario 1 and the UrbanSim scenario file will specify the scenario 4 database as the ``scenario-data''.

Any table in a scenario database ``hides'' the same-named table in the scenario's parent database.