## 3 Methodology and Materials
### Study Area  
Jurong City (118 deg57 deg-119 deg22 deg E, 31 deg56 deg-32 deg13 deg N), located in central Jiangsu Province along the southern bank of the Yangtze River, serves as the paradigmatic case for this research. Geographically positioned between the regional cores of Nanjing and Zhenjiang (Figure 4), Jurong exemplifies a typical interstitial urban area within the polycentric urbanization framework of the Yangtze River Delta. Administratively subordinate to Zhenjiang, Jurong has strategically aligned its development trajectory toward Nanjing through the \"Integration into Nanjing, Coordination with Zhenjiang\" regional policy, reflecting both the city of Nanjing's expanding metropolitan influence and local political-economic ambitions driven by the political achievement tournaments in China [56; 57]. Its strategic location presents a distinctive interstitial spatial configuration, characterized by three key functional roles in Jurong:  
* Metropolitan Integration Zone (Gain access via [[https://www.jurong.gov.cn/jurong/c100278/202111/f0835b2](https://www.jurong.gov.cn/jurong/c100278/202111/f0835b2) fd71a452b9b75f9 ecb3482 cc.shtml]([https://www.jurong.gov.cn/jurong/c100278/202111/f0835b2](https://www.jurong.gov.cn/jurong/c100278/202111/f0835b2) fd71a452b9b75f9 ecb3482 cc.shtml), accessed on 20 February 2025): As the eastern gateway of the Nanjing metropolitan core, Jurong serves as an experimental pilot area for absorbing decentralized urban functions from the urban core of Nanjing City, facilitating metropolitan-scale functional integration. * Regional Exchange Hub [58]: Reflecting its central role within the _Ning-Zhen-Yang Integrated Development Plan (2018)_, Jurong is located at the intersection of Nanjing, Zhenjiang, and Yangzhou. It is officially designated as an anchor city along the Ning-Zhen Development Axis. * Spatial Frontier for Metropolitan Expansion: Jurong is situated approximately 40 km northeast of the central business district in Nanjing (Xinjiekou CBD), within a 45 min commuting radius, positioning it as a critical frontier for the suburban growth and spatial extension of Nanjing. These roles encapsulate the complex identity characterized by administrative fragmentation yet strong functional integration in Jurong, providing an ideal setting for analyzing the spatial externalities inherent to urban interstitial zones. To quantitatively capture real-world spatial interactions and mobility patterns between Jurong, Nanjing, and Zhenjiang, this study utilizes anonymized mobile phone signaling data obtained from the \"Wisefoot\" (Smart Steps) big data platform of China Unicom. The dataset encompasses a one-month period (1-30 November 2019) and comprises spatial location records from mobile devices within the jurisdictions of Jurong, Nanjing, and Zhenjiang. The raw mobile signaling data were systematically processed into structured origin-destination (OD) travel information following a rigorous three-step methodology (outlined in Figure 5). * Core user identification: Initially, we filtered signaling data to define the core population in Jurong. A core user was operationally defined as a mobile device that registered presence within the administrative boundaries for more than ten days within the analysis period (November 2019) in Jurong. This threshold ensures that the identified core users are stable residents or regular commuters rather than transient visitors or incidental passersby. Spatial location data were aggregated onto a standardized grid structure of 1 km \(\times\) 1 km resolution, balancing spatial precision and computational feasibility. Users identified as present for ten days or fewer were classified as transient visitors and excluded to minimize analytical noise from occasional or short-term mobility events. * Derivation of home, work, and other destinations: Subsequently, we derived primary home locations, workplaces, and other significant activity locations for each core user.