City Digital Twins: their maturity level and differentiation from 3D city models

The emerging field of City Digital Twins has advanced in recent years with the help of digital infrastructure and technologies connected to the Internet of Things(IoT).However,the evolution of this field has been so fast that a gap has opened in relation to systematic reviews of the relevant literature and the maturation of City Digital Twins on an urban scale.Our work bridges this gap by highlighting maturity in the field.We conducted a systematic literature review with bibliometric and content analysis of 41 selected papers published in Web of Science and Scopus data-bases,covering five areas:data types and sources,case studies,applied technologies and methods,maturity spectrum,and appli-cations.Based on maturity indicators,the majority of the reviewed studies(90%)were at initial to medium stages of maturity(up to element 3),most of them focused on 3D modelling,monitoring and visualisation.However,digital twins cannot be limited to 3D models,monitoring and visualisation,for they can be developed to include two-directional interactions between humans and com-puters.Such a high level of maturity,which was not found in the reviewed studies,requires advanced technologies and methods such as cloud computing,artificial intelligence,BIM and GIS.We also found that further studies are essential if the field is to handle the complex urban challenges of multidisciplinary digital twins.While City Digital Twins extend by definition beyond mere 3D city modelling,some studies involving 3D city models still refer to their subjects as City Digital Twins.Among the research gaps we identified,we’d like to highlight the need for near-real-time data analytics algorithms,which could furnish City Digital Twins with big data insights.Other opportunities include public participation capabilities to increase social collaboration,integrating BIM and GIS technologies and improving storage and computation infrastructure.