As the digital revolution deepens and pervades every aspect of daily life, virtual realities begin to penetrate one another in a multiplicity of ways. The amount of sensing data being compiled on the city grows, enabling the construction of virtual realities that can, in turn, be transformed for diverse purposes. Here, Michael Batty and Andrew Hudson-Smith from the Centre for Advanced Spatial Analysis, University College London, outline how they went about the construction of a virtual city in central London. A conventional 3-D-GIS /CAD model was used as the basis on which to build a digital realm in which designers are cast as avatars and populations as agents, so as to define new ways in which to understand and plan the city.
Virtual London: Navigating and analyzing in 3-D using GIS, CAD and Multimedia The most obvious of virtual cities is based on the geometry of the city that we represent as streets and buildings and compose in layers. An image of London model, which includes 45,000 buildings or blocks over 20 square kilometers, is shown in Figure 1. Building starts with a digital terrain model – the bare earth. Onto this is draped an aerial photograph. We then lay out the Digital Street and parcel map, then extrude the parcels to their average heights using LiDAR
data – clouds of x-y-z points defining the third dimension generated from low-flying aircraft using lasers that scan the geometry of the city as they fly across it. Figure 2(a) shows an example of this for St Paul’s Cathedral. We can then populate this basic-skeleton model (the building blocks) with data, ranging from building populations, air pollution along the streets, financial data such as rents and property taxes, social conditions such as crime rates, analyses of the impact of tall buildings in terms of locations from where they can be seen, the energy associated with building masses, employment, diversity of building use, and so on. We can also embed within the model other multimedia – for example, digital panoramas that record the ‘real’ detail of the city more superficially, yet also more directly – at any point where such detail might enhance the experience. And of course we can render each building in as much detail as we like, as demonstrated by the fish-eye view in Figure 2(b).

Examples of the applications described above are illustrated in Figure 3, in which we have ‘flooded’ the model with new layers of data. The figure shows how a visualization of pollution, based on the particulate nitrogen oxide (mainly associated with vehicle emissions), can be layered as a surface onto the geometry. It also shows what would happen if the sea level rose 10 meters, which is equivalent to a rise in sea level in the North Atlantic if the Greenland ice cap were to melt. However, more traditionally the model can be used to assess the visual impact of tall buildings on the surrounding area. In Figure 4 (see pp 42–3) we replace Norman Foster’s new Swiss Re building (the ‘Gherkin’) in the City of London with a composite New York skyscraper, illustrating the impact of such a change in scale. Using a 3-DGIS, we can compute the view sheds from every place to any other, thus assessing the impact of relaxing the low-buildings policy that has dominated central London for the last 50 years.

This traditional virtual city model, conceived now as a 3-D view of a large spatial database, can be connected back to the sensing of data in real time – for example, to the air-pollution monitors used to generate using new forms of distributed computing such as the ‘grid’. But we can also connect the digital model back to more traditional icons. Just as we can print a paper copy from a GIS, or a static 3-D image from a CAD model, we can print a hard copy of the digital city in material terms.
in Figure 5, we have printed a little bit of Virtual London (an area around the Gherkin) using a CAD/CAM printer. This took two days to print or, rather, ‘mill-out’, but in the future this type of operation will become routine. Connecting peripherals such as sensing and printing devices (the most obvious being web cams) to the digital model is thus becoming more routine. Once we have such a structure, we can then port it to other worlds and it is to these that we now turn.


Fig.5 Little bit of Virtual London