In the context of the rapid growth of global population, how to achieve high quality and sustainable development of high-density urban living environment is an important topic in the theory and practice of future urban planning. This paper focuses on the cognitive and practical process of urban density in the context of urban planning, reviews the origin and historical process of urban density research, sorts out the evolution stage and topic composition of density research in the process of urbanization development, and looks forward to the frontier of density research for local planning applications, in order to provide references for China’s urban research and urban development under the background of new urbanization.

As a public intervention in the built environment, urban planning consists of two main components: planning compilation and development control, while density (development intensity) control is one of the core indicators of development control. This paper briefly describes the essential features of urban density zoning control, which can be summarized as generality and particularity, as well as hieratical and adaptive. Generality refers to common rules that applies to almost all areas of the city, while particularity refers to specific rules involved in some areas of the city that require local amendments to density zoning controls. The practice of cities at home and abroad shows that urban density zoning control is based on general regulations of development intensity control according to land use types and location conditions, with local specificity reflected through local modifications. The hierachy of urban density zoning control includes total urban development at the macro level, density zoning at the meso level, and development intensity allocation at the micro-level; while adaptability implies the adaptation both in spatial and temporal dimensions. In the context of China’s current new urbanization, urban density zoning control will play an increasingly important role in achieving high-quality urban development, implementing national policy guidance and shaping urban characteristics

Density zoning is a planning and management system that divides urban space according to different development intensities. After 20 years of practice, Shenzhen density zoning has formed a multi- level management control method system for urban development intensity from the macro, meso, and micro perspectives. This paper expounds the basic concepts of urban density and density zoning, summarizes and analyzes the main characteristics of the four versions of density zoning in Shenzhen over the past 20 years, and conducts a comparative analysis. It points out that as a systematic planning and control system, density zoning needs to be guaranteed by strong technical means and management capabilities; density zoning has a strong value orientation and public policy attributes, and its management standards are characterized by relativity and locality. Management objectives should be adapted to the stage of urban development, and management methods should be adapted to the political system and governance system.

In the academic community, there have long been a number of experiential summaries about the scale-related aesthetic rules of urban built environment. However, what the summaries lack is an analytical strategy for further interpretation. This explorative research therefore develops a method that adopts convex spaces as the analytical objects to interpret their particular pattern of scaling laws in those urban open spaces developed with the aesthetic rules. First, the research highlights the importance of the convex spaces and systems as a spatial reflection of people's activity and perception, and a critical link between graphic details and synchronic experience in the urban built environment. Based on this, the investigation examines, analyses and compares the convex space systems derived respectively from two groups of cases. The first group includes three cases of traditional cities acknowledged as good historical examples evolved with a bottom-up order. The second group involves three modern cases shaped by top-down planning. The discrepancy in the scale variation of the convex space systems between the two groups of cases are identified. The result shows that the sizes of the convex spaces are coherently diverse and their distribution is well-defined by exponential laws in the former three cities while they become disjointed in the latter ones. It also illustrates the factors of these spatial features from a systematic perspective. Meanwhile, compared with the existing theories and methods, the examination of convex space systems is argued to be a new effective approach to understand the aesthetic rules of urban forms, and may bring us new insights for urban regeneration practice.

The concept of digital twin has developed rapidly in recent years and has achieved certain results in aerospace, machinery manufacturing and urban areas. This paper uses the ecological system theory to understand the digital twin city, and takes the digital twin city as a system which is composed of five scales: cells, tissues, organs, systems, and living bodies. However, with the development and practice of digital twin cities, it is still under exploration to realize real-time monitoring and data fusion between urban systems, and to support multi-dimensional intelligent analysis and decision-making in public services, economic industries, infrastructure and planning management. Since scale is the basis of cognitive complex systems, the subjects and rules of digital twin cities at different scales are also different, which can result in different design priorities for twinning practices at different scales. Therefore, this paper firstly understands the digital twin city from the system theory, studies the digital twin city as a whole. Second, it is divided into four scales: cell, organ, system, and living body; then we suggest to build a digital twin city scale system framework that conforms to the system theory. At last, three suggestions for building the system framework are put forward: to clarify the differences between subjects and rules at different scales; to provide technical solutions for design requirements at different scales; to ensure collaboration between multiple scales so as to achieve the goal of improving urban operation efficiency and residents' living comfort through digital twin city construction.