Chapter 2: City digital twins (CDTs): A reviving trend

Arguably, the technical essence of CDTs is not new. A CDT is an urban simulation model, which is essentially a mathematical representation of certain urban development processes. Some models incorporate an explicit spatial/geometrical dimension, while others do not, subject to the specific design and purpose of the model. Prior to CDTs, urban modelling and the derivative ‘urban analytics’ (Batty, 2019) has been an established field of academic research and urban policy practice since the 1960s. Sectoral, specialist models have been developed for various urban infrastructures such as transport, energy and water. Most models aim to simulate either short-term system performance and operations for management purposes or mid-to-long term responses to certain policy and/or control interventions for planning purposes. As a high-level categorisation, models can be grouped into descriptive models and causal and/or predictive models. The key difference between the two is that causal relationships are essential for the latter. Geometrical models of cities (usually for visualisation purposes) without the capability of predicting are typically descriptive models. For further discussions on modelling urban systems, see Batty (2009) and Landis (2012).