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Part III. Appendix 2: Data Transportform, while in the 3-d case this is not necessary. In addition to the added complexity that wouldhave to be built into the client, additional metadata would be needed to describe the inventory andassociated 2-d data objects in the first case and the 3-d data object in the second.Requisite Data Transport System Capability #4: It must provide the metadata neededto transform the data to a consistent semantic form, or it must be capable of delivering the data ina consistent semantic form, or both.Syntactic consistency of data sets does not imply their semantic consistency:• variables might be the same, but units of measurement might differ, for example, one data setmight be in cgs units while another might be in mks units, or• the naming conventions used to describe variables might differ among data sets, for example, seasurface temperature might be notated as “sst,” “SST,” “sea surface temperature,” “ts,” “Ts,” “surfacetemperature,” and so on.Semantic meaning can be communicated through translational use metadata, descriptive usemetadata, and search metadata. For the level of interoperability desired, all are required.It ranges from difficult to impossible, however, to envision all the descriptive use metadata thatmay be required of the IOOS system in the future. Different user communities will likely requiredifferent collections of descriptive use metadata, and they may require them in different forms.The Data Transport Team suggests that semantic metadata be kept logically separate from syntacticmetadata. Special attention should be given in the design of the system to providing a flexiblestructure for the semantic use metadata. The resulting system is likely to produce a variety of usemetadata models all sharing a common base.Implementation as an Extensible Suite of ModulesThe Data Transport Team proposes implementing IOOS data transport as a suite of modules basedon a set of transport and semantic protocols addressing the requirements identified above (formattranslation, structural reorganization, and metadata consistency). These modules would be layeredon top of modules that implement TCP/IP protocols and would constitute, in effect, an applicationlayer over TCP/IP.146