OpenCities Map provides the ability to join the properties of selected features through a common attribute. The idea is similar to a database join. For example, a property feature could be joined to a zoning feature if both features have a common property like ZoneType.

Joining features in this way makes the properties of the other feature available whenever the selected feature properties are reviewed or browsed.

Properties to be joined must be of the same type. That is, only integer properties may be joined to integer, double to double, and so on. Joins are saved in the Map Definition in which they are defined.