Visitors may ask for information using obsolete geographical terms from a previous generation, and which do not correspond to current boundaries. In other words, the precise semantics of a geographical term can depend on the time-period in question. Accordingly, we entered street names and other geographical terms from different Ordnance Survey maps between 1880 and 1994. A geographical term in the model refers to one of those four time periods, and to the relationships in place at this period. Relationships exist to express the redefining of an older term by a new one and the removal of a term.
Later terms inherit the attributes of earlier terms.
Queries can be made for changes that have taken place in a given area, within a time frame. The source of geographical data for different time periods and the number of time periods to use is an interesting issue. We entered data by hand from the four different editions of Ordnance Survey maps in our small scale system. Automatic means for creating the model would be necessary for large scale systems. Efficient implementation of the linking of geographical terms to time periods remains a topic for further investigation.