classes GM_Object and GM_Curve specified in ISO 19107 (Figure 12). This allows the subclasses specific to this schema to be used as feature attributes in compliance with the General Feature Model specified in ISO 19109. The second level of the hierarchy consists of a set of classes that describe a one-parameter geometry. These might be used to describe the movement of a feature with respect to any single variable such as pressure, temperature, or time. The third level specializes these classes to describe motion in time. The classes are specified fully in Clauses 6 and 7.