Assessment of Urban Space Green Use Efficiency based on the three-dimensional urban sprawl perspective

Rational and effective spatial utilization represents a critical strategy for cities to address the constraints of limited land resources. While previous studies have developed a two-dimensional perspective framework, we are the first study to adopt the three-dimensional perspective indicator, urban building volume, to delicately characterize urban sprawls in efficiency measurement. We select 274 prefecture-level cities in China as case studies and validate this newly proposed concept with high-resolution building height datasets and socio-economic data. The complete research framework includes Urban Space Green Use Efficiency (USGUE) measurement, identification of the spatial differentiation driving factors, and the significance of the influencing factors. Based on the Slack analysis of the Super-Efficiency Epsilon-Based Measure (Super-EBM), we compare the impact of "two-" and "three-dimensional" space inputs on efficiency analysis. Our results indicate that three-dimensional sprawls exhibit divergent trends compared to two-dimensional sprawls. Urban building volume performs better than built-up area in quantifying efficiency differences. Variables related to the level of development of the city's built-up land, urban vitality, and natural topographic features are the main driving and influencing factors on USGUE. Notably, central and northeastern cities, with similar built-up area size compared to other cities, feature lower building heights and USGUE. Adequate attention to the harmonization of horizontal expansion and vertical development would effectively enhance USGUE and increase urbanization quality.