Answering a need for topographic features
The user may express a need for topographic features (e.g., houses and small paths on Ile-de-France). These features are represented in the data, set but the user representation of geographic space may be different from those used in the geographical data sets. Corona and Winter  demonstrated these differences in the specific application domain of pedestrian navigation.
During the discovery process, the catalog service should interpret the terms “small paths” and “houses” into the metadata base, as illustrated in Figure 4.5. Moreover, the catalog should do this for various users and various data sets.
In this process, the catalog service should rely on a model in which a user may express which topographic features he is interested in. Having such a model for various possible users is a very difficult task, because there is no obvious model of geographic space that everybody would agree on. Instead, users communities build models adapted to their viewpoint. This issue is widely referred to as the definition of a geographical ontology. An ontology is a formal model of a domain that is shared by experts in this domain. Unfortunately, no ontology of commonsense geography is available, and the extensive research in this area failed to propose one.
The catalogs service also needs a list of features represented in the cataloged data sets to determine which data sets contain a representation of the needed features. This description is available in the ISO19115 model as MD_FeatureCatalog Description.