The basic class for dealing with data in Opus is the class Dataset (Section 7.1). A dataset is a collection of attributes for a particular type of entity, such as a set of grid cells, or a set of households. Each member in this set has the same set of characteristics, such as income of household. In Opus, these characteristics are called attributes. Attributes can be either read from a data store (primary attributes) or computed using an Opus variable definition (computed attributes).
Any Opus attribute (primary or computed) can be used as an indicator, although of course only some attributes will be particularly useful as indicators. The primary attributes of interest are commonly in the database tables for the given Opus application. For UrbanSim, these database tables and their attributes are documented in Chapter 9, ``UrbanSim Database Tables.'' Each computed attribute is defined by an Opus variable definition.
Indicators can be computed and visualized, via a GUI5.2 or Python script 5.3. Before describing these in detail, a number of different ways that attributes can be specified needs to be described; specifically, attributes can be specified through its fully-qualified name, as an aggregation of other attributes, or as an expression involving multiple attributes.