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Part III. Appendix 2: Data TransportSEMANTIC METADATAIt is useful to distinguish among use metadata, metadata required to use a data set and typicallytransmitted with data, and search metadata, required to find data of potential interest and typicallyassociated with the directory level.This distinction is quite important because most metadata discussions center around searchmetadata requirements and do not use metadata requirements, for example, the Directory InterchangeFormat (DIF) of the Global Change Master Directory (GCMD). Use metadata and searchmetadata overlap, but one is not a subset of the other, for example, missing value flags are not requiredwhen searching for a data set, while to use the data such information is crucial. Similarly,the ranges of the variables in a data set are not required to use the data, but they form the basis formany data set searches.Use metadata may be further subdivided into translational and descriptive use metadata. The formerrefers to operations that are performed on the data values, be they the names of the variablesor the digital numbers associated with them, that are required for the user to understand theirmeaning. For example, the variable T maps to sea surface temperature or d x 0.125 maps to °C,where d is the number stored in the data set. Descriptive use metadata, on the other hand, tellsabout the data —how the instrument was calibrated or what sensor was used.Search metadata may also be further subdivided, in this case into parameter, range, and descriptivesearch metadata. Parameter search metadata contain the list of parameters or variables in thedata set. This could be further subdivided into dependent and independent variables. Range searchmetadata contain the ranges of variables within the data set. In most existing directory systems,only the ranges for time and space are included. Descriptive search metadata contain other informationassociated with the data set, such as a generic description of the sensor used. There may beoverlap between this descriptive information and those contained in descriptive use metadata, althoughthis need not be the case. For example, a description of the sensor may be relevant to bothgroups, but there is no reason to include detailed information about sensor calibration as descriptivesearch metadata.Figure 1 shows the different metadata types schematically. Although three levels are shown in thisfigure, the inventory and data levels have been treated together. This is the approach that has beentaken in OPeNDAP (i.e., inventory information is treated in the same way as data).169