## 3 Results and Analysis  
### Ecological Quality Evaluation of Sanya  
From 2014 to 2018, the mean RSEI in Sanya decreased from 0.656 to 0.632, indicating a decrease in the overall ecological quality (Table 3). The NDVI and Wet, which had a positive impact on the ecological environment of Sanya, increased from 0.744 to 0.749 and from 0.587 to 0.603, respectively. For the remaining two indicators that negatively affected the ecological environment, the mean value of the LST exhibited a decreasing trend, while the mean value of the NDBSI increased. Although the increases in the NDVI and Wet and the decrease in the LST indicated an improvement in the ecological environment, the increase in the NDBSI made a greater contribution to the ecological damage than the first three, resulting in a decrease in the overall RSEI. This verifies that the ecological quality of Sanya was damaged by the increase in the artificial buildings and bare land due to urban development and construction. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Year** & **NDVI** & **Wet** & **NDBSI** & **LST** & **RSEI** \\ \hline
2014 & 0.744 & 0.587 & 0.514 & 0.537 & 0.656 \\
2018 & 0.749 & 0.603 & 0.526 & 0.391 & 0.632 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The values of ecological indicators and RSEI. The RSEI results were divided into five categories according to the RSEI values using equal intervals: excellent (0.8-1.0), good (0.6-0.8), medium (0.4-0.6), poor (0.2-0.4), and very poor (0.0-0.2) [32]. As can be seen from Figure 2, the areas with excellent ecological quality in Sanya were mainly located in the mountains in the northern part of the Tianya District and on the Luhuitou Peninsula in the south. The mountain ranges bordering the Haitang and Jiyang districts had a high degree of forest coverage and were less affected by human activities. The areas with poor ecological quality were mainly located in the built-up areas in Yazhou Bay town and in the central part of the city, as well as in the Haitang Bay National Coastal Recreation Park. The percentages of areas with excellent, good, and medium ecological environment qualities in both periods were >87%, indicating that Sanya had a good ecological environment background (Table 4). From 2014 to 2018, the excellent RSEI grade area decreased by 132.2 km\({}^{2}\), mostly changing to the good and medium grades. The area of poor and very poor ecological quality increased by a total of 22.9 km\({}^{2}\), so the overall ecological quality of Sanya deteriorated. The areas with an obvious decline in ecological quality were the coastal areas of the Haitang District and the Ningyuan River Basin in the Yazhou District. Based on the RSEI results, it was found that due to the construction of the Science and Technology Industrial Park in the Yazhou District and the development of hotels and real estate driven by the tourism development along the Haitang Bay National Coast, the construction land area in these two areas increased significantly, leading to decreases in the regional NDVI and Wet and increases in the LST and NDBSI. This was the main reason for the decrease in the overall RSEI in Sanya (Figure 2). The mountainous and ecological conservation areas in the northern part of Sanya, on the Luhuitou Peninsula in the south, and in the mountain ranges at the junction of the Haitang and Jiyang districts had relatively high elevations and were mainly covered by forest. The four indicators were relatively