This text shows how the principles and technologies of object-oriented programming, distributed processing and internet protocols can be embraced to further the reliability and interoperability of datasets for the professional GIS market. The book describes the central concept of the interface specification between the data consumer and producer - the Virtual Data Set VDS. It then examines how VDS deals with two other classes of model - field representations and modelling uncertainty. The final part of the book looks at implementation, describing how the VDS interacts with PostScript, Java, and Object-oriented modelling environments.