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Part III. Appendix 2: Data Transportplished directly from the originating server; it might simply constitute a link in the acquisitionchain. In addition, modules in this layer should provide the ability to aggregate data from multiplesites into new data sets that are structurally and semantically consistent. It is clear that some of theoperations performed by modules in the structure layer will be discipline-neutral while others willeither add semantic content to the data or make use of semantic information to structurally reorganizethe data. For example, all gridded data that involve space and time might be required to berepresented as a four-dimensional array of (Longitude, Latitude, Depth, Time); in such a case, if agiven data set were initially organized as (Latitude, Longitude, Time), it would be reorganized tothe preferred 4-d form by inserting a null Depth dimension. As a matter of design philosophy andto provide the most flexibility in the use and evolution of the Data Transport System, operationsthat can be performed in a discipline-neutral fashion will be separate from those that require a semanticunderstanding of the data, certainly logically and probably when implemented by enclosingthem in distinct structure layer modules.The Semantic Layer and the Semantic Data Model—To make use of data sets storedin a heterogeneous interoperable system, at least some semantic metadata must be available; moreover,they also must be consistent (or the ability to map them to a consistent form must exist), andit must also be possible to add new metadata terms as the need arises. The semantics implicit in thestructural transformations that the system may provide and the semantic information transportedin the data access protocol together define the semantic data model. The core of a semantic datamodel is the set of translational use metadata, which must be well defined and easily extensible.Interoperability within the system must be defined to depend only upon this core; that is, whilethe system must be able to carry additional metadata, interoperability within the system must notdepend on the presence of such additional information. Note that the semantic core need not becollocated with the data that it describes, nor does it have to be collocated with the additional semanticmetadata. The system infrastructure must, however, be capable of appending such metadatato the data stream on request. IOOS must be capable of providing access to metadata in a varietyof forms to take advantage of the metadata developed by different communities and it must be capableof providing access to metadata from a site other than that of the data server.Required and Optional Routes through the Layers—The most fundamental operationperformed in the system outlined above is format translation from the storage format to theformat expected by the client. This means that the data access protocol related to the transformationof the data must lie at the core of the system. Other operations might be performed on thedata as they move from originating server to end client but they are not, in general, required.143