Answering a need for data
When a user expresses a need for data (e.g., vector data on Ile-de-France, detailed and exhaustive), the properties relevant to him may be different from these documented in the metadata. The catalog service has to translate user-defined properties into standard metadata. This is illustrated in Figure 4.4.
The difference may be in terms of range domain of the data properties. In the above example the property vector corresponds to the value “001” of the metadata element MD_DataIdentification.spatialRepresentationType. On Ile de France corresponds to the following values of the MD_DataIdentification.Extent.boudingPoly- gon: any polygon that contains the polygon “pIdF,” where pIdF would be the boundaries of Ile de France expressed in the same spatial reference system as that used in the metadata base. The spatial extent could also be expressed in the metadata by a geographic entity or a bounding box.
Translating user-defined values into constraints on the corresponding metadata elements may rely on contextual databases, as already used by catalog services (see Section 4.3.1). This has been experienced in the project LaClef , where the Seamless Administrative Boundaries in Europe (SABE) database is used to map the user area of interest with a metadata constraint.
The difference can also be in terms of elements. For instance, the properties detailed and exhaustive should be mapped with several ISO19115 elements during discovery. The element MD_DataIdentification.SpatialResolution can be expressed as an equivalentScale or as a distance. The user is here looking for the largest scale
or equivalently the smallest distance. The element MD_DataIdentification.topicCat- egory lists the themes of the resource content. The user is possibly looking here for the resource for which the list is the longest. He can also use the MD_FeatureCatalog Description.featureTypes element, which lists features contained in the representation. The date, MD_DataIdentification.exTemp, is also a relevant metadata here because of the exhaustiveness requirement.