Dear Colleagues,

This special issue derives its title and origin from the 12th International Symposium on Digital Earth (ISDE12), on 6–8 July 2021 (https://digitalearth2021.org). Scheduled within the annual "GI-Week", this special event will bring together policymakers and scientists and explore pathways towards the vision of a "Digital Earth". ISDE12 aims to support the UN Sustainable Development Goals (SDGs) by harnessing the world's data and information resources to digitally represent our planet, and to monitor, measure, and forecast natural and human activities on Earth. However, this Special Issue is not only limited to the interested attendees of the conference but is open to all contributors who have relevant works to submit. 

The digital twin concept is the latest technology push dominating the smart city framework. While the virtual image of an object or entity from the real world has already been around for years in engineering, this approach has just recently entered the realm of urban management and city planning. Thanks to the continuing trends toward digitalization and more powerful data processing, analytics and storage capabilities, city administration, and urban planning are breaking new ground with the capability to make more informed, transparent, and effective decisions by integrating and visualizing data from across the urban space. Thus, digital twin technology represents a major step toward becoming a digital metropolis fit for the future. The urban environment we live in is becoming more and more complex. Interrelations and dependencies as well as the effects of changes are becoming increasingly difficult to assess. This is why the digital twin concept is of key importance: It allows creating an image of the reality on which one can effectively test and simulate the effects of new solutions, plans or system changes in the digital image first, before they are then implemented—or showing an effect—in the real world.

Hence, knowledge and insights gained from digital twins promise to make cities more livable, resilient and—as the platform can also be made available to citizens via public interfaces—more inclusive and creative. Digital twins accelerate development processes and act preventively against problems that are difficult to foresee. Ultimately, working with a digital image of the city can help to save costs through prior simulation and to significantly improve the quality of life for citizens. In particular, the overarching goal of an urban digital twin city is to provide a digital copy of the city that contains sensor and data networks, models, simulations, and algorithms reflecting the properties and behavior of the real entities and processes as accurately as possible. This digital sphere enables decisions that influence outcomes in the physical world, forming a positive feedback loop between the physical twin and the digital one—a process that requires long adaptation cycles between real object and digital image. However, this does not mean that decisions and feedback mechanisms necessarily need to be automated. The digital twin may support insights that lead to good decisions. The decisions are then separately enacted in the physical world, and the digital twin is updated through sensor feeds and other in situ measurements or data collections. In practice, a digital twin of a smart city goes beyond the simple 3D or 4D visualization of the urban environment. Rather, it is envisaged as a platform for the development of applications that provide insight in the current state and dynamics of the city by means of performance indicators (e.g., related to energy consumption and supply, local climate, air quality, health, mobility), and simulating—in the virtual world—the effect of possible future scenarios indicating potential follow-up measures. Here, cities can also become more democratic by having a vision of what is lacking in each community, improving their environment and their services. With open data and application interfaces based on common standards, basically anyone can access and develop apps for the platform and therewith create bottom-up solutions.

The increased amount and variety of remote sensing data play a key role in the generation of urban digital twins. On the one hand, they allow the generation of a 3D replica of the built environment down to the finest detail, which in turn enables an immersive experience for the users through augmented, virtual, or mixed reality techniques. On the other hand, they facilitate the derivation of spatiotemporal thematic layers supporting—either alone or in combination with other sources of information such as IoT, social media, or socioeconomic data—the development of the abovementioned applications.

In this framework, this Special Issue encourages submissions related to the conceptualization, development, implementation, and employment of digital twins supporting sustainable and smart cities, with an emphasis on the remote sensing component. Here, topics of interest also include but are not limited to:

• Visualization issues and challenges;
• Collective discovery of knowledge from multiple data sources;
• Virtual, augmented, and mixed reality facilitating knowledge discovery;
• Spatiotemporal modeling across reality-virtuality;
• Anomaly detection;
• Immersive analytics;
• Experience from actual case studies.

Dr. Mattia Marconcini
Dr. Thomas Esch
Guest Editors

Manuscript Submission Information

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