Topographic gradient characteristics of rural settlements in China in recent 40 years

Spatial layout and terrain have undergone marked changes in rural settlements over 40 years, as urbanization expanded rapidly. However, previous studies rarely focused on the topographic gradient differences of rural settlements in China. Systematic and clear descriptions of these regularities remained lacking. In this study, the slope and elevation spectra were calculated at the national, regional (four major economic zones), and county levels in 1980, 1990, 2000, 2010, and 2020. Land use remote sensing and digital elevation data of rural settlements were employed. The terrain gradient characteristics were detected based on digital terrain model. The slope factor was analyzed to construct the terrain slope spectrum and the rural-settlement slope spectrum, while the elevation factor was used to generate the terrain elevation spectrum and the rural-settlement elevation spectrum. Standard deviation ellipse analysis was employed to identify the spatial distribution and gravity center shifts. While the digital terrain analysis was applied to construct the comparative slope and elevation spectra between rural settlements and overall terrain. The results show that: (1) The rural settlements were expanded from 118 201 km² to 145 041 km² from 1980 to 2020, indicating the steady growth with the outstanding regional differentiation. The distribution was characterized by the more settlements in the east and the fewer in the west, with the higher levels of aggregation in the east and greater dispersion in the west. The standard deviation ellipse indicated that there was the persistent northeast-southwest orientation, and the gravity center migrated northwestward by 12.886 km. (2) Rural settlements exhibited a persistent low-slope preference at the national scale. The slope spectrum of rural settlements intersected with the terrain slope spectrum at 1.5°, defining slopes below this threshold as the dominant distribution zone. However, there was a gradual expansion toward the higher slopes, with the strongest climbing tendency in 2000–2010. At the regional scale, the intersections of rural-settlement and terrain spectra were highest in the Western China and lowest in the Central and Eastern China, indicating the outstanding topographic differences. The intersections of rural settlements and terrain slope spectra presented a pattern of Central (1.30°) < Eastern (1.60°) < Northeastern (2.10°) < Western (3.10°) China, indicating that rural settlements were predominantly distributed below these slopes. At the county scale, the slope-climbing intensity increased over time, with the strong climbing concentrated in western highlands under policy and infrastructure support. While the most eastern counties maintained weak climbing trends. (3) In terms of elevation spectrum, the national average elevation of rural settlements increased by 9.805 m from 1980 to 2020. Regionally, the settlement and terrain spectra intersected along an east–west gradient, with low-elevation plains prevalent in Eastern and Central China, slightly higher elevations in Northeast China, and Western China constrained by a high plateau base. The intersections of elevation spectra showed an east–west gradient of Eastern (82 m) < Central (102 m) < Northeastern (265 m) < Western (780 m), implying that rural settlements were mainly concentrated below these elevations. At the county scale, most counties showed no vertical rise, with intermittent elevation uplift observed in Tibet, northern mountainous areas, and southeastern hilly areas, suggesting phased and regionally differentiated expansion. Overall, the multi-scale long-term enlargement of rural settlements showed a shift toward slightly steeper slopes under a persistent low-slope preference, and a modest rise in the mean elevation with the concentrated growth in middle-elevation areas. Topographic gradient constraints should be incorporated to optimize rural settlements planning. Therefore, these findings can support to the urban and rural integration, coordinated revitalization, and risk-aware land-space management.