## 4 Results
### Coupling Coordination Analysis
1.2 Analysis of Spatial Characteristics of Supply-Demand Coupling Coordination Degree of Civil Aviation Passenger Transport  
This study uses ArcGIS 10.8 to draw a spatial variation of supply-demand coupling coordination degree of civil aviation passenger transport in 31 province-level administrative divisions (excluding Hong Kong, Macao, and Taiwan) in China in 2004, 2009, 2014 and 2019 (Figure 5) and calculates the proportion of supply-demand coupling coordination level of civil aviation passenger transport in 2004, 2009, 2014 and 2019 (Figure 6). From 2004 to 2019, the coupling coordination degree of supply-demand of civil aviation passenger transport in various province-level administrative divisions increased year by year, but there was a large spatial heterogeneity. In 2004, the coupling coordination level of provincial civil aviation supply and demand was not extreme coordination. Only Shanghai and Beijing were in moderate coordination. Guangdong, Hainan and Fujian were in low coordination. A total of 83.97% of the province-level administrative divisions were extreme incoordination, which was the main type of coupling coordination level in this period. In 2009, Shanghai was in high coordination, Beijing was in moderate coordination and Guangdong, Hainan, Zhejiang, Yunnan, Sichuan, Fujian, Shandong, Tianjin, Hubei, Shaanxi, Liaoning, Chongqing, Jiangsu, Henan and Xinjiang were in low coordination. A total of 48.39% of the province-level administrative divisions were in low coordination, which was the main type of coupling coordination degree in this period. The remaining province-level administrative divisions were extreme incoordination. In 2014, Shanghai and Beijing were moderate coordination, Hainan and Guangdong were in the moderate coordination stage and Sichuan, Yunnan, Fujian, Zhejiang, Tianjin, Shaanxi, Liaoning, Shandong, Chongqing, Xinjiang, Jiangsu, Hubei, Hunan, Xiziang, Ningxia, Henan, Inner Mongolia, Guangxi, Qinghai, Guizhou and Heilongjiang were in low coordination. The remaining 19.35% of the province-level administrative divisions were in extreme incoordination. In 2019, the coupling coordination degree of supply-demand of provincial civil aviation passenger transport increased significantly. No province-level administrative division was in extreme coordination and all province-level administrative divisions were in the low-level coordination stage and above. From 2004 to 2019, the distribution of highly coordinated, moderately coordinated and low-coordination province-level administrative  
Figure 4: Evolution curve of supply and demand coupling coordination degree of civil aviation passenger transport. divisions showed a spatial expansion trend, and the distribution of extremely unbalanced province-level administrative divisions showed a spatial convergence trend. In this process, the provincial civil aviation passenger transport supply-demand coupling coordination degree level was upgraded to the upper level. The coupling and coordination levels of civil aviation passenger transport supply-demand have been improved to varying degrees in different province-level administrative divisions. This is due to China's large-scale investment and planning in civil aviation infrastructure. The construction of regional airports has improved inter-regional traffic connectivity. The civil aviation industry responds to changes in market demand by optimizing capacity supply and improving service quality. Thus, the level of supply and demand coupling coordination is improved. Figure 5: Spatial variation of supply–demand coupling coordination degree of civil aviation passenger transport in 31 province-level administrative divisions (excluding Hong Kong, Macao, and Taiwan) in China in 2004, 2009, 2014 and 2019. Figure 6: Proportion of supply–demand coupling coordination level of civil aviation passenger transport in 31 province-level administrative divisions (excluding Hong Kong, Macao, and Taiwan) in China in 2004, 2009, 2014 and 2019.