## 3 Results  
### Spatial Range of the PRD Urban Agglomeration Identified by NTL_LandScan (NTL_LS) Fusion Data  
The fusion of NTL data with LandScan data resulted in the NTL_LS dataset (Figure 5). The figure indicates that high-value areas of NTL_LS are primarily concentrated in the core cities of the PRD urban agglomeration, including Guangzhou, Shenzhen, and Foshan, reflecting active economic activities and high population density in these cities. Additionally, the continuous brightness distribution along the Guangzhou-Shenzhen corridor demonstrates a significant economic and population clustering effect. Secondary high-value areas include Dongguan and Zhongshan, which exhibit strong economic connections and relatively high population density, indicating that they play a secondary core role within the PRD urban agglomeration. Low-value areas are mainly located in peripheral cities such as Jiangmen, Zhaoqing, and Huizhou, suggesting relatively weaker economic activities and lower population concentration in these regions. Furthermore, high-value distributions are sparse in the northern and western regions of the PRD urban agglomeration, such as parts of Zhaoqing and Qingyuan, reflecting lower economic and population densities.  
The spatial range of the PRD urban agglomeration identified by the NTL_LS data is shown in Figure 6, with a total identified area of 6069.51 km\({}^{2}\), primarily concentrated in the central and eastern parts of the study area, including Guangzhou, Shenzhen, Foshan, and Dongguan. These cities serve as economic centers of the PRD urban agglomeration, characterized by a developed industrial base, a highly dense population, and strong resource-attracting capacity. The high brightness and population density in these areas reflect active economic activities and strong agglomeration effects. This spatial clustering is attributed to the economic advantages, industrial clusters, and well-developed urban infrastructure in these cities, which attract substantial employment opportunities and a mobile population, further driving the growth of surrounding areas.  
In the mid-eastern cities of the PRD urban agglomeration, such as Dongguan and Foshan, the proximity to core cities, coupled with relatively high economic and population densities, creates tight economic connections and an integrated urban network with Guangzhou and Shenzhen, establishing these cities as essential components of the PRD  
Figure 5: Fusion of NTL Data with LandScan Data.  
Figure 6: Spatial Range of the PRD Urban Agglomeration Identified by NTL_LS Data.  
urban agglomeration. This network effect reinforces spatial clustering within the region, forming a multi-core structure within the urban agglomeration. In contrast, the western and peripheral regions of the PRD urban agglomeration, including Jiangmen, Zhaoqing, and Huizhou, while encompassed within the agglomeration's spatial boundaries, exhibit lower economic and population densities, resulting in less spatial coverage identified in the NTL_LS data. This characteristic may be influenced by factors such as regional industrial structure, infrastructure development levels, and the strength of economic connections with core cities. Geographic distance and transportation accessibility further contribute to the relatively weak spatial clustering of these peripheral cities. Notably, ecological protection zones in the western PRD urban agglomeration, including wetlands and mountainous areas, also constrain spatial development in this region, impacting the spatial expansiveness of the urban agglomeration.