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Understanding the 3D spatial structure of a city is increasingly essential for addressing various environmental and management issues, surpassing the importance of traditional 2D analysis. However, existing studies neglect the diversity of building height and still lack a clear description of 3D urban structure. This article proposes a new framework to uncover the 3D urban structure. Firstly, kernel density is employed to reveal the hierarchical spatial structure of buildings and the contour tree method is improved to quantitatively measure the spatial diversity and complexity. Then, the 3D urban structure is abstracted by spatial interpolation after feature filtration. Finally, this framework is applied to the central area of Chengdu City, revealing that: (1) Spatial structures of buildings with different heights exhibit significant diversity, location preference, and complexity; and (2) A globally "depression" 3D urban structure with low center—high periphery is obviously identified. This framework provides an effective way to reveal the 3D urban structure in a more intuitive and clearer way from various disordered urban buildings, which can be transferable to other cities and further facilitate sustainable planning and development of cities. [ABSTRACT FROM AUTHOR]

Three-dimensionality (3D), the main feature of urban morphology since rapid urban development, has been a trending topic on urban morphology research. Many 3D morphological indices have been proposed to describe urban morphology in current studies. However, the understanding of those indices is independent and isolated, and the spatial relationship of these indices is rarely considered, which makes it difficult to grasp the overall morphological characteristics and its spatial structure. Based on 3D buildings in the central area of Chengdu City, nine morphological indices are constructed. Then, a spatially constrained multivariate clustering method is applied for the comprehensive regionalization of urban morphology after index correlation inspection. The results show that: (1) four morphological indices from height, density and volume can effectively describe the characteristics of urban morphology in the study area respectively; (2) similar to the landform map, a ten-region map is generated that can clearly and intuitively demonstrate 3D morphological zones of integrated characteristics; and (3) a "two-ring with multi-sector" spatial structure is further abstracted from the regional map. This study contributes to acknowledging the spatial structure of urban morphology from a vertical perspective, and the framework can be reduplicated in other cities, which are meaningful to urban management, planning, and construction. • The spatial relationship of 3D morphological indices is considered. • A synthesized regional map is generated to show different 3D characteristic zones. • A "two-ring with multi-sector"spatial structure is abstracted from the vertical perspective. [ABSTRACT FROM AUTHOR]