Detecting Hillside Urbanization Process Through the Fusion of Nighttime Lights and Landsat Data: A Case Study of Kunming City

Rapid urbanization has transformed the spatiotemporal transformation of urban landscape patterns in Kunming City, resulting in the formation of a substantial number of hillside urbanization landscapes. Hillside urbanization has alleviated the conflict between urban development and the protection of flat land resources. However, excessive hillside development may disrupt the ecological balance in mountainous areas such as Kunming. This study integrates the Suomi national polar-orbiting partnership visible infrared imaging radiometer suite-like (SNPP-VIIRS-like) data and Landsat satellite imagery to propose a modified nighttime lights urban index (MNUI) to extract urban construction land. The results show that: (1) Compared with the vegetation-adjusted nighttime lights urban index (VANUI), normalized difference urban index (NDUI), and enhanced nighttime lights urban index (ENUI), the MNUI has a higher extraction accuracy, with an overall accuracy (OA) of 91.8–93.8%, indicating its superior accuracy in extracting urban construction land; (2) between 2000 and 2023, the area of urban construction land in Kunming City expanded more than threefold, from 149.27 to 484.49 km², primarily through edge-expansion and infilling expansion; (3) over the same period, the area of hillside urbanization increased from 13.18 to 64.39 km², an almost fivefold increase, and the average slope of urban construction land rose from 2.64 to 3.03°, reflecting this increase in hillside urbanization; and (4) it is projected that the area of hillside urbanization will reach 72.06 km² by 2030. This study is expected to provide scientific references for urban construction planning and sustainable development strategies in Kunming.