Digital Terrain Analysis Based On Different Scales And Resolutions DEMs

With the wider application of GIS, the study on digital terrain analysis is becoming a matter of concern for more and more people.Digital Elevation Model, which plays an important role in soil, disaster prevention and many other areas, is the most important space information and core data systems in digital terrain analysis. The terrain information capacity of DEM with different scales and cell sizes exists remarkable differences.As we known, DEM with the high-resolution can include more detailed topographic information,but in many practical applications not all the studies need high-resolution elevation data grid, because too detailed data may even case inconvenience in numeration as data redundancy. So in this paper, we selected Low Mountain and Hilly Region of Chongqing city as research area, based on DEM with 1:10,000, 1:50,000, 1:100,000, 1:250,000 scales and different resolutions, using spatial analysis, superimposition analysis, mathematical statistics, then extracting altitude, slope, aspect, curvature, amplitude, ravine density, Stream-power indices, Topographic wetness indices and Terrain characterization index, analyzing and comparing on the terrain information. According to the study, the main contents and results are as follows:(1) The comparison of topographic information of DEM with different scales.The reduction of scales somewhat has generalize effect on terrain information.The results of 1:10,000 scale is considered as the real value, comparing the maximum slope value with other three scales DEM, the slope loss rate of 1:50,000, 1:100,000, 1:250,000 scales DEM are 25.14%, 10.80%, 46.73%, respectively. The reduction of scales could make the area of flatness region in aspect analysis increase gradually. The maximum increment is on 1:250,000 scales DEM, reached 621.58km~2, about 1/3 in the region. However, on the whole, the areas of the other aspects are rather even.The reduction of scales synthetize contour line on topographic diagram.And it reduce to a large extent the profile curvature, while uneven distribution of the degree increases, then it lead to the distribution curve of information more steep.There is a similar trend in the analysis result of the plan curvature that the scale become smaller can make the area increase at the plane of bigger curvature.The whole area more concentrated in a small plane curvature level.We select a suited size of the window of the study area experiencing several test in order to extract the information. Gaining results from the analysis: area of hill of the scale 1:10000 account for 87.55% of the total area; area of highland distributing between 75m and 200m occupy 60.83% of the scale 1:50000, while 0.3% which lie in the area of plain of slightly undulation and moderate mountain. Minishing in scale shows its advantages are mainly in representing macroscopical topographty.There are Some differences when extracting the information of hvdrography with minishing of scale. Form the figure of extracting ravine from four different scales of DEM, it is clearly seen that the minishing of scale aggravates the neglect of ravine, therefore density of ravine gradually decreases. The results from DEM of 1:10000 are as 7.34 times as that of 1:250000.Topographic Index can reflect special distribution of soil water, saturation of land surface and the process of creating of runoff, at the same time the spatial distribution shows the spatial distribution of soil saturation or lack of. Through calculating secondary topographty index of different scale DEM, it can infer that the mean value of index of valley power and topographty humidity will increase with decreasing of the scale, SPI of the two increases from 7.36 to 8.17, also distribution around mean value in scale 1:50000 account for 53.11% of the total, while that in scale 1:100000 account for 45.33%. There are obvious differences in the results of topographty feature index, the change scope of scale 1:50000 are from-185.17 to 104.45.(2)The comparison of topographic information of DEM with different cell sizes.Resulting from slope analysis: as cell size is bigger the resolution is lower, and along with that its max-slpoe, mean-slpoe, min-slpoe and standard value minished. Moreover, in allusion to 1:10000 DEM, in the slope level of 0～3°, when cell size changed from 5m to 25m, the area gradually minishes too. When the cell size becomes 100m, the information loses seriously, and the max-slope is only 41.55°. The augment of the cell size will cause physiognomy detail character becomes simple; in the comparison of the different cell size, most area differences appear in the flat region. The different in the flat region under the resolution of the 100m cell size DEM and the 10m cell size DEM achieve 481.58km~2.The reduction of DEM resolution makes the area of the section curvature distribute more concentrated in the low curvature. Taking the 1:10,000 scale DEMs as the example.at the section curvature 0~3°,when the the grid space resolution is 10 meters,the minimum value of area is 228.31 km~2. When the grid space resolution is 40 meters and 50 meters, the section curvature area increases evidently.When the grid space resolution is 50 meters, the value reaches to a 984.87km~2.Lower gradually along with the resolution, the loss of the plane curvature also presents the trend of increasing.Different resolution digital elevation model have significant effects on the topography. Taking the 1:250000 scale as the example, as the resolution debases, the relief amplitude increases. When we choose the grid with 100m, Analysis window area is 5000m×5000m, and the minimum of the ups and downs of the value is 44.77m, Plain-regional ups and downs of the area is decreasing in trend. Compare with the Gully density we can see the impact by the different resolution on was not significant.As a 1:250,000-scale measure five different resolution DEM ravine density the range is 2.91 km/km~2～1.06km/km~2.