How Does the Built Environment Shape Urban Vitality Across Multiple Scales? A Nonlinear Comparative Analysis of Chengdu and Chongqing in China

The built environment is the core material carrier shaping urban vitality, and its impact on urban vitality constitutes a key research hotspot in urban geography and urban–rural planning. Most existing studies focus on single cities and single scales. They pay insufficient attention to the heterogeneity of their relationship across different city types and spatial scales. They also lack a systematic framework for multi-dimensional comparative analysis. This study takes Chengdu and Chongqing as cases. They are the core cities of the Chengdu–Chongqing Twin-City Economic Circle. Three grid scales are applied. Using the XGBoost–SHAP-integrated model, this paper explores the differences in indicator importance, nonlinear impacts, and threshold effects of built environment on urban vitality. The objectives of this study are as follows: (1) This study will reveal the spatiotemporal differentiation characteristics and patterns of urban vitality across multiple cities, multiple grid scales, and multiple time periods. (2) This study will identify the relative importance of built environment indicators and their heterogeneous patterns across different cities and grid scales. (3) This study will clarify the nonlinear relationship between the built environment and urban vitality, as well as grid-scale differences and city differences. The results show the following: (1) Urban vitality exhibits significant distribution differences across cities, grid scales, and times. (2) In terms of relative importance, mean building height and building density are both important influencing indicators of urban vitality at multiple grid-scales in different cities. The effects of certain built environment indicators on urban vitality vary across cities and grid scales. Road intersection density plays a prominent role in Chengdu, while commercial accessibility has a significant influence in Chongqing. As the scale changes, indicators including road density, road intersection density, and commercial accessibility demonstrate distinct variation patterns. (3) The nonlinear effects of the built environment on urban vitality are significant and differ across cities and grid scales. The nonlinear effects of certain built environment indicators in Chongqing are more complex than those in Chengdu. As the scale changes, the nonlinear effect trends and thresholds of certain built environment indicators also show significant variations. Based on multi-city and multi-scale spatial analysis, this study deepens our systematic understanding of the relationship between the built environment and urban vitality. It provides a quantitative basis for understanding the interaction between human activities and physical spaces in different types of cities and at different grid scales. It also provides a referable paradigm for multi-dimensional analysis in similar studies.