Database Tables about Geographies

Note: the tables described in this section are only needed for the SQL indicators, which is no longer used within CUSPA. You do not need to create these tables if you are only creating indicators via the standard mechanisms of using the Opus infrastructure to view attriutes of Opus variables.

Each user of UrbanSim may have different types of geography that are of interest to them. Some of them are standard and required for the model, such as traffic analysis zones and urban growth boundaries. These geographies impact the way the model works. Others geographies are useful for computing summations, aggregations, and cross-tabulations. These may be arbitrary and specific to that metropolitan area, such as small districts and large districts.

The geography tables described here provide a way to have both these standard geographies and an unlimited number of arbitrary geographies as defined by the user.

A geography is an arbitrary coverage over part, or all, of the metropolitan area being simulated. Each geography consists of a set of polygons. Using the geographies table together with the verb|gridcells_in_geography| table allows the user to specify which grid cells are in which polygon of a given geography.

Cells Partially within a Polygon. The issue of grid cells partially within a polygon should be handled in a consistent way for the data for a given region being modeled. Two reasonable decision rules are to include a cell in a given polygon if its centroid is inside the polygon, or to include a cell in a given polygon if at least 50% of its area is within the polygon. Use whichever rule is most convenient (the geographies mechanism itself doesn't demand that any particular rule be used).

To add a new geography:

1. Add a new row to the geographies table.
2. For each combination of grid cell and geography polygon in this geography for which the grid cell is within the polygon, add a row to the gridcells_in_geography table. These rows typically are the result of using a GIS tool to overlay a shapefile on the grid-cell grid. See cells partially within a polygon regarding what to do with cells partially within the geography polygon of interest - for example, if the cell-centroid decision rule is used, add the appropriate row to the table if the cell's centroid is within the polygon.