Error in digital network and basin area delineation using d8 method: A case study in a sub-basin of the Ganga

Digital elevation model (DEM) is one of the input data derived from different satellite sensors for hydrologic and hydraulic modelings. Two prime questions could be answered before using these DEMs. First, the acceptability of datasets for our use and second appropriate resolution of the dataset. Three widely used DEMs SRTM 30m, ASTER 30m and SRTM 90m are analyzed to evaluate their suitability to delineate river network and basin boundary area. The hydrology tool of spatial analyst extension inbuilt in ArcGIS 10.2 (which uses the D8 method for calculation of flow direction) has been used for the delineation of both river networks and basin boundary. The assessment of river network alignment and boundary delineation is carried out in the seven sub-catchments of Gandak river basin having different morphological characteristics. The automatically delineated boundary area for all the three DEMs reflects a significant difference when compared with the digitized basin area from the Ganga flood control commission (GFCC) map. The maximum boundary area delineation error is 39137.20 km₂ forASTER 30m, and minimum delineation error of 13239.28 km₂ for SRTM 90m. In the stream network, delineation accuracy is good for SRTM 90m while, except Gandak trunk, ASTER 30m DEM shows better delineation accuracy indicated by mean absolute error (MAE) and standard deviation (SD).     

Comparison of digitally delineated stream networks from different spaceborne digital elevation models: A case study based on two watersheds in South India

Digital elevation models (DEMs) are widely used in the studies related to hydrological modeling. It provides spatially distributed topographical information which helps in the extraction of stream networks. Currently, DEMs obtained through spaceborne remote sensing platforms are increasingly utilized for stream network extraction. However, it is necessary to understand the quality of the derived products and their related issues. Therefore, a comparative study based on stream network extraction in geographical information system (GIS) is performed on such DEMs for two different terrains. Performance metrics such as root mean square error (RMSE), mean absolute error (MAE), standard deviation (SD) of error, correctness index (CI), and figure of merit (FM) are used for the DEM evaluation. The DEMs with finer spatial resolution performed better than coarser resolutions in all the performance indices. It is also observed that the derived stream network from the DEMs having same 30-m spatial resolution (ASTER and CartoDEM) has varying degree of errors. The positional errors of the streams are found to be less for the regions with steep slopes compared to flat terrain.     

空间分辨率与取样方式对DEM流域特征提取的影响

随着数字水文的兴起和分布式水文模型研究的发展, 利用DEM提取水文特征, 进而进行水文模拟的方法越来越广泛地为水文学者所采用. 空间分辨率的改变与DEM重新取样方式对水文模拟都会产生重要影响. 采取不同取样方法获得多种尺度的DEM, 对不同分辨率下的流域特征值进行了统计分析与比较, 引入熵的概念度量不同分辨率的DEM包含的信息量, 以及不同取样方式对信息量的影响. 并计算了以50 m DEM所包含的信息量为基准, 在不同的信息损失下所要求的最低分辨率.     

Uncertainty of soil erosion modelling using open source high resolution and aggregated DEMs

Digital Elevation Model (DEM) is one of the important parameters for soil erosion assessment. Notable uncertainties are observed in this study while using three high resolution open source DEMs. The Revised Universal Soil Loss Equation (RUSLE) model has been applied to analysis the assessment of soil erosion uncertainty using open source DEMs (SRTM, ASTER and CARTOSAT) and their increasing grid space (pixel size) from the actual. The study area is a part of the Narmada river basin in Madhya Pradesh state, which is located in the central part of India and the area covered 20,558 km². The actual resolution of DEMs is 30 m and their increasing grid spaces are taken as 90, 150, 210, 270 and 330 m for this study. Vertical accuracy of DEMs has been assessed using actual heights of the sample points that have been taken considering planimetric survey based map (toposheet). Elevations of DEMs are converted to the same vertical datum from WGS 84 to MSL (Mean Sea Level), before the accuracy assessment and modelling. Results indicate that the accuracy of the SRTM DEM with the RMSE of 13.31, 14.51, and 18.19 m in 30, 150 and 330 m resolution respectively, is better than the ASTER and the CARTOSAT DEMs. When the grid space of the DEMs increases, the accuracy of the elevation and calculated soil erosion decreases. This study presents a potential uncertainty introduced by open source high resolution DEMs in the accuracy of the soil erosion assessment models. The research provides an analysis of errors in selecting DEMs using the original and increased grid space for soil erosion modelling.     

http://www.sciencedirect.com/science/article/pii/S1674987114000036

The paper evaluates sensitivity of various spaceborne digital elevation models （DEMs）, viz., Advanced Spaceborne Thermal Emission and Reflection Radiometer （ASTER）, Shuttle Radar Topography Mapping Mission （SRTM） and Global Multi-resolution Terrain Elevation Data 2010 （GMTED）, in comparison with the DEM （TOPO） derived from contour data of 20 m interval of Survey of India topographic sheets of 1 ： 50,000 scale. Several topographic attributes, such as elevation （above mean sea level）, relative relief, slope, aspect, curvature, slope-length and -steepness （LS） factor, terrain ruggedness index （TRI）, topo- graphic wetness index （TWI）, hypsometric integral （lhyp） and drainage network attributes （stream number and stream length） of two tropical mountain river basins, viz. Muthirapuzha River Basin and Pambar River Basin are compared to evaluate the variations. Though the basins are comparable in extent, they differ in respect of terrain characteristics and climate. The result.,; suggest that ASTER and SRTM provide equally reliable representation of topography portrayed by TOP（） and the topographic attributes extracted from the spaceborne DEMs are in agreement with those derived from TOPO. Despite the coarser resolution, SRTM shows relatively higher vertical accuracy （RMSE -- 23 and 20 m respectively in MRB and PRB） compared to ASTER （RMSE - 33 and 24 m） and GMTED （RMSE - 59 and 48 m）. Vertical accuracy of all the spaceborne DEMs is influenced by relief of the terrain as well as type of vegetation. Further, GMTED shows significant deviation for most of the attributes, indicating its inability for mountain-river-basin-scale studies.     

Post‐Palaeozoic uplift history of southeastern Australia revisited: Results from a process‐based model of landscape evolution

Digital elevation models (DEMs) are widely relied upon as representations of the Earth's topographic morphology. The most widely used global DEMs available are ETOPO5, TerrainBase and JGP95E at a 5‐arc‐minute spatial resolution, and the GTOPO30 and GLOBE (version 1) global DEMs at a 30‐arc‐second spatial resolution. This paper presents the results of intercomparisons of these global DEMs over Australia, and with the GEODATA 9‐arc‐second DEM (version 1) of Australia. These DEMs were also compared to an independently produced, altimeter‐derived orthometric height database. This allows not only a totally independent assessment of the quality of these different DEMs over Australia, but also an insight into the ERS‐1 radar altimeter's ability to measure orthometric heights on land. The results of all these comparisons reveal large differences among the DEMs, with the greatest difference between JGP95E and ETOPO5 (mean 49 m, standard deviation ±274 m). The comparison with the altimeter‐derived database shows good agreement with the version 1 GEODATA DEM (mean 2 m, standard deviation ±27 m), thus demonstrating that the altimeter is a viable method for quality assessment of DEMs in lowland regions. A further conclusion is that the representation of the Australian land surface in both the JGP95E and TerrainBase global DEMs is more accurate than the higher resolution GLOBE (version 1) global DEM, even though JGP95E displays a disparity along the 140°E meridian because of the different data sources used in its construction.     

Automated extraction of watershed boundary and drainage network from SRTM and comparison with Survey of India toposheet

The recent development of digital representation has stimulated the development of automatic extraction of topographic and hydrologic information from digital elevation model input, using geographic information system (GIS) and hydrologic models that integrate multiple databases within a minimal time. The objective of this investigation is to compare the drainage extracted from Shuttle Radar Topography Mission (SRTM) data with the drainage digitized from topographic data (1:50,000) and also to draw attention to the functions of an add-on tool in ArcGIS 9.2 (Arc Hydro v.2) of Kuttiyadi River basin. The analysis reveals that the watershed extracted from the SRTM digital elevation model (DEM) (90 m resolution) is having an area of 668 km² and that from toposheet is 676 km². The river mouth in the drainage network from the SRTM DEM is found to be shifted to the northern side from where it actually exists. The drainage network from SRTM DEM at stream threshold 15 (0.0002 % of maximum flow accumulation) is delivering best results than the other threshold value in comparison with the drainage pattern derived from toposheets. The study reveals the importance, reliability, and quaintness of drainage network and watershed derived from the SRTM using the Arc Hydro Tool, an extension for Environmental Systems Research Institute ArcGIS. The advantage of the Arc Hydro Tool is that it would help a novice with little GIS knowledge to run the model to obtain watershed and drainage network.     

The importance of digital elevation model resolution on granular flow simulations: a test case for Colima volcano using TITAN2D computational routine

The mobility of gravity-driven granular flows such as debris flows or pyroclastic density currents are extremely sensitive to topographic changes, such as break in slopes, obstacles, or ravine deviations. In hazard assessment, computer codes can reproduce past events and evaluate hazard zonation based on inundation limits of simulated flows over a natural terrain. Digital Elevation Model (DEM) is a common input for the simulation algorithm and its accuracy to reproduce past flows is crucial. In this work, we use TITAN2D code to reproduce past block-and-ash flows at Colima volcano (Mexico) over DEMs with different cell size (5, 10, 30, 50, and 90 m) in order to illustrate the influences of the resolution on the numeric simulations. Our results show that topographic resolution significantly affects the flow path and runout. Also, we found that simulations of past flows with the same input parameters (such as the basal friction angle) over topography with different resolutions resulted in different flow paths, areas, and thickness of the simulated flows. In particular, the simulations performed with the 5- and 10-m DEMs produced similar results. Also, we obtained consistent simulation results for the 30- and 50-m DEMs. However, for the coarser 90-m DEM results are largely different and inaccurate. We recommend generating a benchmark table in order to acquire characteristic values for the basal friction angle of studied events. In case of rugged topographies, a DEM with high resolution should be used for more confident results.     

平原河网地区汇水区的划分方法——以上海市为例

借助DEM提取汇水区是分布式水文模型计算和面源污染计算的前提条件.在平坦地区直接使用DEM提取出的水系、汇水区边界与现实情况差异很大.针对现有汇水区提取方法存在的问题,在RIDEM模型的基础上提出了一种平原河网地区汇水区划分的方法,该方法通过将道路、建筑物、水系、沟渠、坑塘等影响径流途径的地物要素融合进DEM,达到细化...     

水文模型系统在峨嵋河流域洪水模拟中的应用

研究目的是采用水文模型系统(HMS)模拟峨嵋河流域暴雨水文过程,并为长江上游地区气候和水文响应研究提供可靠的信息。HMS是一种分布式水文模型可用于研究各种气候因子和地表覆盖变化而引起的水文过程响应,该系统(HMS)利用气象、土壤类型、土地利用和地表覆盖、数字高程(DEM)和降雨径流等资料,研究气候、陆面、地表水和地下水的相互作用机理。在本次研究中,采用SCS Curve Number(CN)和Green-Ampt(GA)方法来计算径流过程,用GIS来数字化DEM、土壤、土地利用和陆地覆盖数据。通过用不同时间间隔的降雨和不同计算方法的水力参数模拟水文过程,来检验降雨的时间尺度效应和水力参数的空间变异性对水文过程的影响。结果表明,HMS对峨嵋河流域暴雨洪水的模拟及预测具有较好的适用性。     

Geomorphometry and fractal dimension of a riverine badland in Maharashtra

One of the most serious limitations in studying the surface morphometry of a badland landscape is the nonavailability of a very fine resolution data which is essential for such types of studies. Local relief of most of the badlands in India and also from other parts of the world exhibit limited relief amplitude, often within a few meters. The paper reports a case study carried out in a riverine badland formed along the Western Deccan Trap Region. An attempt has been made in the present paper to extract the morphometric variables of the landscape from the DEMs derived from a high resolution field generated data, because the accuracy of the DEM derived values are dependent on the pixel resolution of the DEM from which they are generated. The size of the pixel resolution should be fixed differently for different landscapes depending on the landscape process in the area. The local relief of the area is around 10 m and for such types of landscapes the topographical maps and also the web-available DEMs are of very coarse resolutions which are not suitable for the analysis. Therefore two well defined tributary catchments were chosen from the area under investigation and theodolite surveys were carried out, contours were generated with 10 cm interval, DEMs were derived by using Arc GIS software. SRTM (Shuttle Radar Topography Mission) 90 m resolution data were utilized to generate DEM for the whole basin. Hypsometric and the drainage basin parameters were extracted from these data by using the same software. Fractal dimension of the whole basin and the sample basins were also obtained for the same data. The morphometric data generated were used to understand the geomorphic processes operating in the area.     

基于数字填图系统的遥感等数据在构造-地层分区和地层单位识别中的应用——以1：25万民和县幅、临夏市幅和定西市幅数字地质填图为例

在甘肃和青海交界地区的1：257Y民和县幅、临夏市幅和定西市幅数字区域地质调查中,以“数字填图系统（RGMAP）”为平台,使用遥感ETM^＋数据和地形数据对岩石地层进行了信息挖掘。数据准备主要是遥感和数字地形多源信息的提取和预处理．从数字地形图的等高线数据中提取高程,进而提取汇水网络等水文信息和地形参数。遥感数据经过地形校正、线性增强、色彩拉伸,进行空间分辨率增强,经彩色合成并结合数字高程,实现了对调查区构造-地层区划和部分岩石地层单元的识别。实践证明该项工作是地质填图的重要辅助手段。     

Modelling of piping collapses and gully headcut landforms:Evaluating topographic variables from different types of DEM

The geomorphic studies are extremely dependent on the quality and spatial resolution of digital elevation model(DEM)data.The unique terrain characteristics of a particular landscape are derived from DEM,which are responsible for initiation and development of ephemeral gullies.As the topographic features of an area significantly influences on the erosive power of the water flow,it is an important task the extraction of terrain features from DEM to properly research gully erosion.Alongside,topography is highly correlated with other geo-environmental factors i.e.geology,climate,soil types,vegetation density and floristic composition,runoff generation,which ultimately influences on gully occurrences.Therefore,terrain morphometric attributes derived from DEM data are used in spatial prediction of gully erosion susceptibility(GES)mapping.In this study,remote sensing-Geographic information system(GIS)tech-niques coupled with machine learning(ML)methods has been used for GES mapping in the parts of Semnan province,Iran.Current research focuses on the comparison of predicted GES result by using three types of DEM i.e.Advanced Land Observation satellite(ALOS),ALOS World 3D-30 m(AW3D30)and Advanced Space borne Thermal Emission and Reflection Radiometer(ASTER)in different resolutions.For further progress of our research work,here we have used thirteen suitable geo-environmental gully erosion conditioning factors(GECFs)based on the multi-collinearity analysis.ML methods of conditional inference forests(Cforest),Cubist model and Elastic net model have been chosen for modelling GES accordingly.Variable's importance of GECFs was measured through sensitivity analysis and result show that elevation is the most important factor for occurrences of gullies in the three aforementioned ML methods(Cforest=21.4,Cubist=19.65 and Elastic net=17.08),followed by lithology and slope.Validation of the model's result was performed through area under curve(AUC)and other statistical indices.The validation result of AUC has shown that Cforest is the most appropriate model for predicting the GES assessment in three different DEMs(AUC value of Cforest in ALOS DEM is 0.994,AW3D30 DEM is 0.989 and ASTER DEM is 0.982)used in this study,followed by elastic net and cubist model.The output result of GES maps will be used by decision-makers for sustainable development of degraded land in this study area.     

Geomorphological reconstruction and morphometric modelling applied to past volcanism

Accurate and precise morphometric modelling can be extended to past eruptions after a careful palaeogeomorphological reconstruction of volcanic landforms in an oceanic island. This reconstruction was used to derive the pre-, post-eruption and present-day digital elevation models (DEM). The correct pixel size, interpolation method and quality of these DEMs are discussed. The process in a Geographical Information System framework of the geological information of cone, lava flow and tephra fall deposits together and the aforementioned DEMs allowed the determination of main morphological features of these volcanic landforms and their derivatives. The calibration and validation of morphometric modelling were performed on simulated volcanic landforms, and verified by a case study: a typical Holocene monogenetic basaltic eruption at Gran Canaria Island (Spain).     

The effectiveness of high-resolution LiDAR data combined with PSInSAR data in landslide study

The spatial resolution of digital elevation models (DEMs) is an important factor for reliable landslide studies. Multi-interferometric techniques such as persistent scatterer interferometric synthetic aperture radar (PSInSAR) are used to evaluate the landslide state of activity and its ground deformation velocity, which is commonly measured along the satellite line of sight (LOS). In order to compare velocities measured by different satellites in different periods, their values can be projected along the steepest slope direction, which is the most probable direction of real movement. In order to achieve this result, DEM-derived products are needed. In this paper, the effectiveness of different DEM resolutions was evaluated in order to project ground deformation velocities measured by means of PSInSAR technique in two different case studies in the Messina Province (Sicily, southern Italy): San Fratello and Giampilieri. Three DEMs were used: (i) a 20-m resolution DEM of the Italian Military Geographic Institute (IGM), (ii) a 2-m resolution DEM derived from airborne laser scanning (ALS) light detection and ranging (LiDAR) data for the San Fratello 2010 landslide, and (iii) a 1-m resolution DEM derived from ALS LiDAR data for the area of Giampilieri. The evaluation of the applied method effectiveness was performed by comparing the DEMs elevation with those of each single permanent scatterer (PS) and projecting the measured velocities along the steepest slope direction. Results highlight that the higher DEM resolution is more suitable for this type of analysis; in particular, the PS located nearby the watershed divides is affected by geometrical problems when their velocities are projected along the steepest slope.     

基于ArcGIS的DEM流域划分

从数字高程模型DEM(Digit Elevation Models)直接提取河网及流域信息,是分布式水文模型开发与应用的基础.本文利用ArcGIS中的水文分析工具从DEM中提取了流域水文特征.主要包括:DEM的预处理、水流方向的确定、水流累积量提取、河网的提取、集水区的提取和流域的划分.本文探讨了两种不同划分流域的方法...     

Identifying environmental impacts of underground construction

Dewatering of the groundwater resource and associated reduced flow of surface water features are potential negative impacts when constructing underground facilities. Little work has been done to develop methods for the early detection of environmental impacts on water resources where major underground construction is being undertaken. Recognizing this, prior to construction of two rock tunnels in the southwestern USA, a 3-year preconstruction program was implemented to monitor over 100 wells, springs, and streams in the project area that might be affected. This preconstruction monitoring phase has established data for a hydrologic reference which indicates a high degree of spatial and temporal variability. This variability must be accounted for when trying to identify construction-related impacts. The project area was subdivided into areas of similar characteristics based on geologic and hydrologic features. Measurements from features within each unit were then normalized and aggregated to derive a single representative flow parameter. This representative flow was then correlated to precipitation and major stream flow records to allow for a method of estimating unimpacted flow and groundwater levels during and after construction. Application of this method proved useful in determining and enabling a quick response to construction-related impacts.     

数字高程模型预处理方法的研究进展

数字高程模型(Digital Elevation Model,简称DEM)是地形表面形态属性的数字化表达,被广泛应用于流域水文模拟中河网水系的提取.从DEM直接提取的河网水系及相关的流域地理空间信息,是分布式水文模拟的地理信息平台.由于DEM中洼地和平坦区的存在会影响水流方向的确定和数字河网的正确提取.因此在河网自动提取过程中必须首先对DEM数据进行预处理.本文对国内外各种DEM预处理方法进行了归纳总结.将DEM数据预处理方法分为两大类:分步处理法,以及一体化处理法.分步处理法按处理对象又可分为洼地处理方法和平坦区处理方法两部分;而一体化处理法则采用迭代算法同时对洼地和平地进行处理.     

Usefulness of adaptive filtering for improved Digital Elevation Model generation

The Digital Elevation Models (DEMs), which represent the variation of elevation in a terrain at spatial level, are an important source of input to a variety of applications for deriving a number of terrain parameters such as relative relief, slope, aspect direction etc. In recent years, Synthetic Aperture Radar Interferometry has been viewed as a powerful approach to derive quality DEMs from a pair of SAR images. Despite the interferometric technique is often limited by several de-correlations several researchers demonstrate its effectiveness in topographic mapping. The DEM accuracy is strongly influenced by the effectiveness of the phase unwrapping technique. In this study an effective adaptive filtering approach has been used to reduce the phase noise due to de-correlation and in improving the accuracy of phase unwrapping. Two well known phase unwrapping approaches such as branch cut and minimum cost flow network have been used. Interferometric data from ASAR sensor onboard ENVISAT satellite have been used. A highly undulated terrain condition near Dehradun city situated in Uttarakhand state of India was selected to investigate the performance of this adaptive filtering approach. The RMS error between the InSAR derived elevations and the map derived elevations was obtained as 7.2 m using adaptive filter. However, elevation map of the study area could not be generated due to high de-correlation effect without the use of adaptive filter. This result clearly demonstrates the effectiveness of adaptive filtering approach for generation of DEM at meter level accuracy, which is sufficient for many engineering applications.     

基于多层DEM与QTPV的混合数据模型及其在地质建模中的应用

提出一种基于多层DEM与似三棱柱体元（QTPV）的混合三维数据模型。模型包含顶点、线段（棱边、三角形边）、三角形、侧面四边形、似三棱柱体元和DEM 6个基本元素,同时包含点对象、线对象、面对象、体对象、复杂对象、空间对象等6个对象。设计了6个基本元素和2种地质对象的数据结构和它们之间的拓扑关系。研究了基于原始采样点和基于内插点的两种建模方法。以模拟数据和地质钻探数据对开发的原型系统进行验证。研究表明,基于多层DEM与似三棱柱体元的混合三维数据模型具有同时表达空间对象的表面和内部结构的能力,适合地质勘探领域的三维建模。