Decoding peri-urban transformation through multi-scale mapping of bidirectional urbanisation.

Rapid urbanisation in emerging economies is reshaping the geography of human settlement at an unprecedented pace, placing acute pressure on the principles of sustainable development. In India, these transformations are particularly pronounced in non-metropolitan regions, where expansion routinely surpasses statutory (de jure) administrative boundaries. Yet conventional analyses of urban sprawl fail to capture these processes' complex, bidirectional nature, providing an incomplete account of the functional and spatial extent of contemporary urban growth. Here, we present a novel, data-driven geospatial framework for delineating dynamic urban expansion boundaries and quantifying bidirectional growth patterns. We apply this methodology to Salem and its surrounding towns in Tamil Nadu, India, spanning the period 2001-2020. Our three-tiered approach combines: (i) buffer analysis to identify the spatial threshold of built-up expansion; (ii) centroid shift analysis to track macro-scale directional change; and (iii) landscape metrics to measure micro-scale fragmentation and aggregation. Over two decades, Salem's built-up area increased by ~ 170%, generating a compact urban core encircled by a fragmented, outwardly expanding periphery. Neighbouring towns, including Omalur, Sankari, Rasipuram, and Vazhapadi, exhibited marked bidirectional expansion towards Salem, underscoring the emergence of strongly interconnected regional growth corridors. By redefining urban boundaries through quantitative spatial metrics, our framework enables early detection of emergent urban linkages and sprawl intensities. These findings not only advance the methodological frontier of urban spatial analysis, but also provide actionable intelligence for planners and policymakers seeking to manage expansion sustainably and equitably in rapidly transforming city-regions.