Dan WU, Hui KONG

Shaanxi Provincial Land Engineering Construction Group Co., Ltd., Xi’an 710075, China; Institute of Land Engineering and Technology, Shaanxi Provincial Land Engineering Construction Group Co., Ltd., Xi’an 710075, China; Key Laboratory of Degraded and Unused Land Consolidation Engineering, Ministry of Natural Resources, Xi’an 710075, China; Shaanxi Provincial Land Consolidation Engineering Technology Research Center, Xi’an 710075, China

Abstract Based on the DEM raster image data with a resolution of 60 m × 60 m in the Sanjiangyuan region, the topographic heterogeneity in the Sanjiangyuan region was analyzed by calculating and processing the topographic relief, elevation standard deviation, terrain heterogeneity and surface roughness indexes.The results show that the overall terrain elevation of the Sanjiangyuan area is high, and the surface undulations are large.The southern and southeastern parts of the region are the places with the most dramatic terrain changes.The absolute values of terrain undulations, elevation standard deviation and surface roughness are large.In the central and western regions, the terrain change is small, the relief degree is small, the elevation standard deviation and the surface roughness are small.

Key words Topographic heterogeneity, Relief of topography, Surface roughness

1 Introduction

For geographical research, terrain analysis is the most important basic research, and it is widely used in geography, ecology, humanities, civil engineering and other related research, generally as the initial research object, so as to provide a basis for further research and analysis.The concept of heterogeneity comes from biology.Heterogeneity in terrain can be simply understood as the degree of terrain change per unit area, and it refers to the different nature and performance of terrain.Since the 18century, contour topographic map has been introduced into topographic research, and it has been applied to this day.It is an important method to study topographic conditions.

The surface of earth is continuous.Landscape heterogeneity research is one of the important data of DEM grid data, and the landscape heterogeneity in DEM data grid unit can be understood as the complexity and variability of terrain grid cell.It can not only describe the morphological variation of terrain in the cell, but also behave as a unit and terrain elevation variation.The research on DEM data terrain analysis at home and abroad is mainly divided into two aspects:(i)the research on the accuracy of DEM data at different scales;(ii)research on the problems caused by the uncertainty of DEM data, that is, the influence of DEM data scale on the accuracy of terrain expression and the influence of DEM scale on terrain analysis and geological model.

The ecological environment of the Sanjiangyuan is primitive and unique, sensitive and complex to the external influence.It is the largest nature reserve in China and also the nature reserve with the highest concentration of biodiversity in the high-altitude areas of the world.Therefore, the protection of ecological diversity in this region is particularly important.

2 Research area and methods

2.1 Research area

The Sanjiangyuan refers to the headwaters of the Yangtze River, the Yellow River and the Lancang River, located in the southern part of Qinghai Province and the hinterland of the Qinghai-Tibet Plateau.It is dominated by mountainous landforms, with long mountain ranges, high terrain and complex terrain, with an elevation of 3 335-6 564 m.The topographic relief is relatively small, and the landforms are all controlled by geological structures.The geomorphology and natural geographical process in this region are relatively "young", the climate conditions are special, and the cold differentiation of the ground is strong, but the influence of human factors on the natural environment is relatively weak, the natural vegetation succession, the evolution of rivers and lakes and other natural processes in this region basically maintain the natural and original state, and the ecological diversity is prominent.

2.2 Research methods

2.2.1

Topographic relief.Topographic relief is an indicator reflecting the longitudinal change of terrain, and refers to the difference between the maximum and minimum elevation difference.It can reflect the difference of terrain change and ground relief in a certain area to a certain extent.

The calculation formula of topographic relief is as follows:

RF

=

H

-

H

(1)

where

RF

refers to the relief of terrain;

H

refers to the maximum elevation value; and

H

refers to the minimum elevation value.

2.2.2

Surface roughness.Surface roughness refers to the deformation characteristics of terrain in the horizontal direction, which is generally defined as the ratio of surface area of surface element to its projection area on the horizontal surface.The undulation refers to the change of the terrain in the longitudinal and vertical directions, while the roughness refers to the change of the terrain in the plane.

2.2.3

Elevation analysis.Standard deviation, a parameter often used in mathematics, can reflect the deviation of some data from the standard data when it is used in the description of topographic changes.Elevation standard deviation describes the variation of elevation and is a measure index of terrain variation in grid cells.Elevation standard deviation reflects the degree of dispersion between elevation of each point and average elevation.

When elevation standard deviation is used to describe the terrain, and when the size of grid unit is close to or consistent with the mountain slope length, elevation standard deviation reflects the elevation variation of the terrain in grid unit.However, when the size of grid unit is not consistent with the length of mountain slope, elevation standard deviation can give the roughness of geomorphic landscape unit.

3 Results and analysis

3.1 Calculation of topographic relief

The processing results of topographic relief in the Sanjiangyuan are shown in Fig.1.As can be seen from the figure, the relief degree in the whole region is relatively large, especially in the eastern and central southern regions, which is shown as bright white in the image.The relief value is large, reaching about 1 700 km in local areas.The terrain drop is large and the heterogeneity is obvious.

Fig.1 Topographic relief processing results in the Sanjiangyuan

3.2 Calculation of surface roughness

As can be seen from Fig.2, the surface roughness calculation results show that the areas with higher curvature values in the east and central south of the region and places with lower curvature values are dense and changeable.The curvature values in the middle and the whole west of the region are relatively single, with no obvious changes in adjacent areas, and the curvature values are close to 0 numerically.This indicates that the degree of concave-convexity in the eastern and central southern areas of the region is variable and the degree of concave-convexity is large, and the terrain is rough, while the degree of concave-convexity in the central and western areas is relatively mild, and the roughness of the terrain is gentle.

Fig.2 Surface roughness treatment results in the Sanjiangyuan

3.3 Elevation standard deviation calculation

The description of elevation changes in the Sanjiangyuan region through elevation contour processing is shown in Fig.3.According to the contour processing results in Fig.3, the elevation contours of the Sanjiangyuan are denser in the east and south, and the changes are large.Therefore, the southeast and south of the study region have large undulations, large slopes, towering mountains, and many canyons and cliffs, and the western, northern, and central areas have high elevation values and belong to plateau mountainous terrain, but the vertical changes of the terrain are small, and the original mountain topography is relatively flat.The central, eastern and southern parts of the study region are mostly huge canyons formed by rivers longitudinally cutting the mountains.The potential energy of the rivers is very rich.With the passage of time, the contour changes in this region will be more obvious; the central, western and northern areas are due to the topography.Compared with flat, melting water from glaciers is not easy to flow out.Natu-ral frozen soil resources have naturally formed many types of wetlands such as swamps and lakes, which are rich in wetland resources.The contour lines in the region are separated by 500 m.The contour lines are dense in the eastern and central southern areas, and the contour lines in other areas are sparse.The denser the contour lines, the more rugged the terrain, the greater the undulation, and the more obvious the heterogeneity.

Fig.3 Elevation contour processing results of the Sanjiangyuan

4 Conclusions

Sanjiangyuan is located in the hinterland of Qinghai-Tibet Plateau, the roof of the world, and the terrain is mainly mountainous plateau.In the east and central south of the region, the terrain heterogeneity, relief degree, elevation standard deviation and surface roughness are large.In the middle and west of the region, the terrain heterogeneity, relief degree and natural conditions are small.The western part of the region has the worst ecological environment and is also the most vulnerable region.