Data services for the grid have focussed so far primarily on virtualising access to distributed databases, and encapsulating file location. However, orchestration of services requires richer information semantics than these mechanisms provide. Service inputs and outputs must be semantically matched, or characterised in order that sensible transformations may be performed. In many domains important information structures must be aggregated across multiple files, and numerous legacy file formats obscure the natural logical structure of information types. We present a solution for constructing semantic data services for an earth-sciences data grid (the UK NERC DataGrid). A semantically-rich data model is developed, drawing on components from external ontologies. A 'storage descriptor' provides the mechanism for mapping legacy file-based storage onto data model instances. Finally, data services may be built on top of the data model to expose a semantic view of the data irrespective of the underlying file storage details. Our approach is similar to wrapper/mediator architectures for integrating database management systems.