## 3 Results and Analysis
### Urban Built-Up Land Expansion Simulation  
The urban spatial pattern of Sanya exhibited a strip-shaped distribution, and the built-up area was mainly distributed in the central part of the city (including the Tianya and Jiyang districts), the Yazhou District, and the Haitang District (Figure 3). Under the policy background of building an international tourism island in 2010, the urbanization process of Sanya has accelerated. In 2014, the area of Sanya's built-up land area was 122.1 km\({}^{2}\), accounting for 6.66% of the city. In 2018, the built-up land area was 171.5 km\({}^{2}\), accounting for 9.35%. The built-up land area of Sanya increased by 49.4 km\({}^{2}\) from 2014 to 2018, with an increase of 40.4% and an annual increase of 12.3 km\({}^{2}\). The trend of urban spatial growth was coastal to inland expansion, and the three regions exhibited a trend of integration and connectivity. Based on the statistical data for Sanya, in 2014, the GDP of Sanya was 40.438 billion yuan, and the population was 585,600. In 2018, the GDP was 59.551 billion yuan, and the population was 614,600. The GDP growth of Sanya from 2014 to 2018 was 47.26%, and the population growth was 5%. The high economic development and population growth were the main reasons for the expansion of the urban area in Sanya from 2014 to 2018.  
The simulation of the UGB of Sanya under the natural growth scenario was based on the land-use change trend from 2014 to 2018 and the PLUS model. The results show that the kappa coefficient and overall accuracy of the simulation results are 0.841 and 89.28%, respectively, indicating that the accuracy of the model simulation is high. Thus, it is suitable for simulating the growth of the urban built-up area in Sanya in 2030. The built-up area in Sanya in 2030 under the natural development scenario increases significantly, and the  
Figure 3: Urban built-up land expansion trend.  
built-up land area reaches 298.0 km\({}^{2}\), accounting for 16.26%, which is 126.6 km\({}^{2}\) higher than that in 2018. Sanya's urban spatial growth mainly occurs in the central part of the city, the Yazhou District, and the Haitang District. The growth of the urban space in the Yazhou District is dominated by expansion in two directions: along the coast of Yazhou Bay, and in the downstream area of the Ningyuan River. The land use classification data show that there is a large amount of cultivated land in this area, so the urban growth occupies the agricultural space. Second, in the area along the coastline of Haitang Bay, the overall building of the region is denser, and the town expansion exhibits a trend of dispersal. The third area is the central part of Sanya, which exhibits a more obvious trend of spreading from the central mountainous area.  
Under the natural growth scenario, the urban built-up area occupies an area of approximately 112.8 km\({}^{2}\) that previously had excellent and good ecological qualities, with an area proportion of 10.28% of the bottom-line ecological pattern of Sanya, which poses a serious threat to ecological security. Therefore, to avoid damaging the ecology and agriculture due to the disorderly expansion of the city, the actual situation of Sanya should be considered, and the bottom-line ecological security pattern determined by the REEI evaluation should be used as rigid boundaries of the urban development to prohibit urban development and occupation.