## 3 Results
### Validation with the Nightlight, Building, and POIs Characteristics  
After extracting the urban-rural fringes, we further validated the results from different perspectives. Many studies have proven that nightlight imagery can reflect socioeconomic levels [56; 57; 58]. Figure 4 shows the differences in the distribution of nightlight values in urban areas, rural areas, and urban-rural fringes. The clustering results revealed distinct nightlight intensity patterns: rural areas showed consistently low values (generally within 1000) due to sparse population; urban areas exhibited high values (generally above 2000, averaging around 4000) due to abundant light sources such as streetlamps and building lights; and urban-rural fringe areas displayed intermediate values (1000-3500) with notable overlaps with urban areas, reflecting their transitional nature. Importantly, these value ranges emerged naturally from our clustering algorithm rather than being predetermined thresholds. The larger overlap between fringe and urban areas, compared to the minimal  
Figure 3: Urban–rural fringe identification results in Beijing. overlap between fringe and rural areas, reflected the gradual urban-rural transition process where fringe zones often contained newly developed areas with urban characteristics. These patterns validated our clustering results and demonstrated the effectiveness of nightlight imagery in characterizing urban-rural transitions. Figure 5 presents the distribution of the building characteristics for the three land types, including the building floor, area, and length. First of all, the value range of the building floors in urban areas and the urban-rural fringe was wide. In general, the median floor of buildings in urban areas was about 5, and there were also some high-rise buildings with more than 10 floors. The urban-rural fringe had a large number of self-built houses with less than four floors and also a small number of medium and high-rise buildings. The overall floor in rural areas was low, and there were no buildings in a large number of areas, with an average number of floors close to zero. Similarly, the distribution of building areas and perimeters in the urban-rural fringe was basically between those of urban and rural areas, which reflected the transitional nature of its land use. It is worth mentioning that compared with urban areas, there were some buildings with a larger area and perimeter but lower number of floors in the urban-rural fringe. We suspect that the reason for this is that the cost of land in the urban-rural fringe is lower, thus land developers are able to build over large areas, and low-rise buildings are generally more economical than high-rise buildings. In addition, many special functional buildings, such as logistics centers, factory warehouses, industrial parks, are also built there in reality, forming a unique building characteristic. Figure 6 shows the proportion of 14 types of POIs among the three land types. The number of POIs of all types in rural areas was lower than that in urban areas and in the urban-rural fringe, which reflected the relative lack of infrastructure in rural areas. The proportion of POIs in the urban-rural fringe and urban areas showed a similar trend, but there were some differences. In urban areas, the density of POIs was greater than that of the urban-rural fringe, except for the density of enterprises. This is in line with the fact that many of Beijing's large industrial parks are usually located not in the center of the city but in the urban-rural fringe between the Fourth and Sixth Ring Roads [59]. Because there  
Figure 4: Distribution of nightlight values for three land types. is more unused land in this area, it is easy to build a large area of parks, which leads to a greater density of enterprises. The number of foods, daily life, shopping, and transport POIs in urban areas was much larger than that in the urban-rural fringe.