## 3 Results
### Spatial Range of the PRD Urban Agglomeration Identified by NTL_Heatmap (NTL_HM) Fusion Data  
The NTL_HM data, generated by fusing NTL data with heatmap data, are shown in Figure 7. The high-brightness areas are primarily concentrated in core cities such as Guangzhou, Shenzhen, Foshan, and Dongguan and their surroundings; Zhongshan, Huizhou, and Zhuhai also display relatively high brightness, though lower than Guangzhou and Shenzhen. In contrast, Jiangmen and Zhaoqing exhibit significantly lower brightness, reflecting weaker economic activity and population mobility in these peripheral cities. Compared to the NTL_LS data, the high-brightness areas in the NTL_HM data more prominently represent the distribution characteristics of population mobility. The continuity of high brightness along the Guangzhou-Shenzhen corridor, in particular, highlights this corridor as not only an economic axis but also a primary channel for population movement. Zhongshan, Huizhou, and Zhuhai show relatively high brightness but lower than Guangzhou and Shenzhen, indicating weaker population mobility. This data fusion approach is more effective for analyzing the spatial patterns of economic activity and population mobility within the region.  
The spatial range of the PRD urban agglomeration identified based on NTL_HM data is widely distributed (Figure 8), covering an area of 9038.47 km\({}^{2}\), particularly concentrated in cities around the Pearl River Estuary, including Guangzhou, Shenzhen, Foshan, and Dongguan. These areas, marked by economic vitality and high levels of urbanization,  
Figure 7: Fusion of NTL Data with Heatmap Data.  
serve as major zones for economic activity and population mobility, reflecting the high concentration of economic and population flows within the PRD urban agglomeration. These cities, as crucial regions for economic development and population concentration, not only play a dominant role in the regional economy but also attract a significant amount of migrant population, providing employment and living opportunities for surrounding areas. Substantial spatial range is also identified in Zhongshan, Huizhou, and Zhuhai, indicating relatively high economic activity and population mobility in these regions. In particular, areas between Zhongshan, Dongguan, and Huizhou near Shenzhen show dense population flows, reflecting close economic ties with cities like Guangzhou and Shenzhen and indicating strong development potential. In contrast, the peripheral areas of the PRD urban agglomeration, such as Jiangmen and Zhaoqing, show a relatively smaller identified spatial range, suggesting lower levels of economic activity and population mobility. Due to their greater distance from the main economic centers, these cities experience a weaker economic spillover effect and thus lack the large-scale population mobility and economic agglomeration seen in core areas, placing them in regions of lower population and economic density within the PRD urban agglomeration.  
Compared to the spatial range of the PRD urban agglomeration identified by NTL_LS data, which integrates LandScan's resident population distribution characteristics, the NTL_LS-identified range emphasizes areas of stable, long-term economic activity and high population density. Consequently, it primarily captures the core areas of high economic and population concentration within the PRD urban agglomeration. By contrasting the spatial range identified by the two datasets, it becomes apparent that cities like Guangzhou and Shenzhen have the highest resident population density and most active economic activity, while areas such as Zhongshan, Huizhou, and Zhuhai exhibit relatively less economic and population mobility, resulting in a smaller identified range. Peripheral cities like Jiangmen and Zhaoqing display even smaller spatial ranges, with lower levels of economic activity and fewer residents, indicating that these areas are in a low-density state regarding resident population and economic agglomeration within the PRD urban agglomeration.