Exploring the spatial and temporal evolution of three-dimensional urban expansion: evidence from three urban agglomerations in China

Understanding urban expansion's spatial and temporal evolution is crucial for sustainable urban development. Previous research primarily focuses on two-dimensional perspectives, overlooking vertical development. This study investigates three-dimensional urban expansion in Beijing-Tianjin-Hebei (BTH), Yangtze River Delta (YRD), and Pearl River Delta (PRD) from 1998 to 2018 using Local Climate Zone (LCZ) data, standard deviation ellipse, compactness indicators, and landscape ecology techniques. Findings reveal that BTH showed minimal variation in expansion direction. YRD predominantly expanded northwest. PRD primarily expanded eastward. Scattered patches with declining density characterize expansion areas. Open lowrise buildings dominate the PRD and YRD, while BTH transitioned from compact to open lowrise buildings. The increase in open lowrise buildings corresponds with rising landscape complexity and declining fragmentation. Open highrise buildings, though dominant, show a gradual decrease. This study offers comprehensive insights into the dynamics of urban expansion, aiding in developing more habitable and efficient three-dimensional cities.