The class GFI_Feature (Figure 2) is an instance of the .og.metaclass.cg. GF_FeatureType (ISO 19109). It represents the set of all classes which are feature types. NOTE GFI_Feature is implemented in GML (ISO 19136:2007) by the element gml:AbstractFeature and type gml:AbstractFeatureType. In an implementation this abstract class shall be substituted by a concrete class representing a feature type from an application schema associated with a domain of discourse (ISO 19109, ISO 19101). Sampling Features (Clause 8) are a class of feature-types whose role is primarily associated with observations.