The website to which the Cambridge University Library website has directed us:

• gives access to numerous historical maps
  
  
• introduces the notion of "added value":
  • it distinguishes between the uses of web images which "serve no useful purpose", and "the imaginative uses of online technology" ;

    e.g. in cartography, a plain map is of much less interest than one with "added value", which, in this field, may mean using multimedia capabilities ("imaginative uses of technology"): having the option of "overlaying" different types of data, which enables (for instance) comparisons between several periods by superimposing variants of state boundaries, roads, etc at different stages of their history.
    "Overlaying of information from different periods, as offered by David Rumsey's GIS Browser. As applied to selected US cities, 'the current geospatial data that can be overlaid and compared to the historical maps includes roads, lakes, parks, state boundaries, digital orthophotos (aerial photography), topographic mapsheets, digital elevation models and satellite imagery.'"
    A GIS (Geographical Information System) is a combination of a digital map and a database, allowing the user to retrieve statistical data corresponding to geographical areas defined on the map by the user with a selection tool.