Urban Vitality Clusters as Activity-Based Diagnostic Units: Spatial Differentiation, Morphological Interpretation, and Planning Translation

Urban vitality is often measured within predefined administrative units, blocks, or grids, limiting its translation into planning-oriented spatial diagnosis. This study examines 34 vitality clusters in the Nanjing Metropolitan Area and develops a multi-scale framework linking cluster identification, spatial interpretability, type stratification, morphological differentiation, micro-scale case examination, and planning translation. The identified clusters show substantial correspondence with built-up territories while extending across administrative boundaries in several cases. Five operational area–density groups are distinguished using study-specific thresholds. Their distribution reveals a gradient from high-density cores to medium-density and peripheral low-density clusters, together with an internal pattern of high-vitality cores and lower-vitality edges. After Benjamini–Hochberg correction, 30 of 60 non-redundant variables remained significant, mainly in functional, transport, and morphological dimensions. Micro-scale cases further show the co-occurrence of smaller blocks, continuous frontages, mixed functions, and favorable walking conditions. These findings suggest that vitality-derived clusters can serve as morphologically interpretable, activity-based diagnostic units that complement conventional planning units in stock-based urban regeneration.