Application of digital elevation model and orthoimages derived from IRS-1C pan stereo data in monitoring variations in glacial dimensions

An attempt has been made to study variations in the glacier extent over a period of time using digital elevation model (DEM) and orthoimages derived from IRS-1C PAN stereo pairs of 1997–98 and topographical map surveyed during 1962–63. DEM and orthoimages have been generated using integrated software developed for processing of IRSIC/ID panchromatic stereo data using the softcopy photogrammetric workstation. Case studies of two glaciers, i.e. the Janapa garang and Shaune garang glaciers of the Basapa basin, a sub-basin of Satluj River in India, have been presented here. Generation of DEM has been followed by the estimation of its accuracy. PAN images were interpreted for identification of the snout of the glaciers. The geographical locations of the snouts on the images were compared with the location as mapped on the topographical map of the study area. To verify satellite observations, field investigations were carried out at Shaune garang glacier area. The Janapa garang and the Shaune garang are observed to have retreat of 596m and 923 m respectively. Reduction in the thickness of ice in the deglaciated part of the Shaune garang glacier was estimated on the basis of change in the elevations of the glacial surface from 1963 to 1998.     

An improved ANUDEM method combining topographic correction and DEM interpolation

Void filling and anomaly replacement in conjunction with auxiliary sources of data have been widely used to improve the quality of existing problematic high-resolution digital elevation models. However, the traditional interpolation methods used for this purpose have always failed to eliminate the discrepancies between different data-sets. In this paper, an improved ANUDEM method is presented for DEM interpolation, which incorporates the idea of topographic correction using high correlation of topological structure between contour lines (CLs) from multi-scale digital elevation models (DEM). Firstly, the terrain topological structure is extracted from the CLs of a low-resolution DEM. The topographic surface correction is then undertaken based on the extracted structure, which recovers the topographic information of the sharp depressions and eminences to fit the high-resolution representation. Finally, the breaklines of the terrain surface are distilled and integrated into the denser DEM generation. The experiments undertaken confirmed the superiority of the proposed method over the other DEM interpolation methods. It is shown that the proposed method can provide results with a higher accuracy, as well as a better visual quality.     

The isotropic organization of DEM structure and extraction of valley lines using hexagonal grid

The automatic extraction of valley lines (VLs) from digital elevation models (DEMs) has had a long history in the GIS and hydrology fields. The quality of the extracted results relies on the geometrical shape, spatial tessellation, and placement of the grids in the DEM structure. The traditional DEM structure consists of square grids with an eight‐neighborhood relationship, where there is an inconsistent distance measurement between orthogonal neighborhoods and diagonal neighborhoods. The directional difference results in the extracted VLs by the D8 algorithm not guaranteeing isotropy characteristics. Alternatively, hexagonal grids have been proved to be advantageous over square grids due to their consistent connectivity, isotropy of local neighborhoods, higher symmetry, increased compactness, and more. Considering the merits above, this study develops an approach to VL extraction from DEMs based on hexagonal grids. First, the pre‐process phase contains the depression filling, flow direction calculation, and flow accumulation calculation based on the six‐neighborhood relationship. Then, the flow arcs are connected, followed by estimating the flow direction. Finally, the connected paths are organized into a tree structure. To explore the effectiveness of hexagonal grids, comparative experiments are implemented against traditional DEMs with square grids using three sample regions. By analyzing the results between these two grid structures via visual and quantitative comparison, we conclude that the hexagonal grid structure has an outstanding ability in maintaining the location accuracy and bending characteristics of extracted valley networks. That is to say, the DEM‐derived VLs based on hexagonal grids have better spatial agreement with mapped river systems and lower shape diversion under the same resolution representation. Therefore, the DEMs with hexagonal grids can extract finer valley networks with the same data volume relative to traditional DEM.     

A Triangular Form‐based Multiple Flow Algorithm to Estimate Overland Flow Distribution and Accumulation on a Digital Elevation Model

In this study, we present a newly developed method for the estimation of surface flow paths on a digital elevation model (DEM). The objective is to use a form‐based algorithm, analyzing flow over single cells by dividing them into eight triangular facets and to estimate the surface flow paths on a raster DEM. For each cell on a gridded DEM, the triangular form‐based multiple flow algorithm (TFM) was used to distribute flow to one or more of the eight neighbor cells, which determined the flow paths over the DEM. Because each of the eight facets covering a cell has a constant slope and aspect, the estimations of – for example – flow direction and divergence/convergence are more intuitive and less complicated than many traditional raster‐based solutions. Experiments were undertaken by estimating the specific catchment area (SCA) over a number of mathematical surfaces, as well as on a real‐world DEM. Comparisons were made between the derived SCA by the TFM algorithm with eight other algorithms reported in the literature. The results show that the TFM algorithm produced the closest outcomes to the theoretical values of the SCA compared with other algorithms, derived more consistent outcomes, and was less influenced by surface shapes. The real‐world DEM test shows that the TFM was capable of modeling flow distribution without noticeable ‘artefacts’, and its ability to track flow paths makes it an appropriate platform for dynamic surface flow simulation.     

念青唐古拉山冰川高程变化的ICESat监测

针对青藏高原冰川高程变化研究较少的问题,该文提出一种大范围区域的冰川高程变化监测方法。基于ICESat激光高度计数据,联合利用SRTM DEM数据,计算念青唐古拉山脉冰川的高程变化,进而反演冰川的冰量变化。结果显示,念青唐古拉山冰川高程在2003—2009年间平均减薄速率为(0.53±0.47)(m·a~(-1)),估算得到冰量年均减少(0.32±0.28)km~3,总体呈逐年减少趋势,证明冰川一直处于消融状态。拉萨和当雄气象站的资料表明,冰川消融主要是由于当地气温升高。     

1:10000DEM生产工艺改进方法研究和应用

归纳了基于ArcGIS平台,利用地形图数据库生产1:10000DEM的工艺流程。并从以下两方面对DEM生产工艺流程进行改进:一、首次提出采用距离权法自动内插生成三维(3D)特征线,有效减少、甚至消除构TIN产生的"平台效应";二、针对生成的DEM数据,文中还提出一种大样本自动采样的精度评价方法,减少人工检查工作量。试验证明改进工艺流程能有效提高作业效率,改善DEM产品精度,使DEM产品更贴合实际地形,可用于DEM数据的快速生产。     

An enhanced approach for surface flow routing over drainage‐constrained triangulated irregular networks

The accuracy and efficiency of the simulations in distributed hydrological models must depend on the proper estimation of flow directions and paths. Numerous studies have been carried out to delineate the drainage patterns based on gridded digital elevation models (DEMs). The triangulated irregular network (TIN) has been increasingly applied in hydrological applications due to the advantages of high storage efficiency and multi‐scale adaptive performance. Much of the previous literature focuses mainly on filling the depressions on gridded DEMs rather than treating the special cases in TIN structures, which has hampered its applications to hydrological models. This study proposes a triangulation‐based solution for the removal of flat areas and pits to enhance the simulation of flow routing on triangulated facet networks. Based on the drainage‐constrained TIN generated from only a gridded DEM by the compound point extraction (CPE) method, the inconsistent situations including flat triangles, V‐shape flat edges and sink nodes are respectively identified and rectified. The optimization algorithm is an iterative process of TIN reconstruction, in which the flat areas are generalized into their center points and the pits are rectified by embedding break lines. To verify the proposed algorithm and investigate the potential for flow routing, flow paths of steepest descent are derived by the vector‐based tracking algorithm based on the optimized TIN. A case study of TIN optimization and flow path tracking was performed on a real‐world DEM. The outcomes indicate that the proposed approach can effectively solve the problem of inconsistencies without a significant loss in accuracy of the terrain model.     

Drainage skeletonization from flow-accumulated area without the use of threshold

This study presents a method of automatic drainage skeletonization from flow-accumulated area without the use of threshold which conserves drainage geometry at chosen digital elevation model (DEM) scale. To get all possible drainage at the chosen scale, stream order raster is generated by incorporating flow accumulation and flow direction raster derived from corresponding DEM. This allows generation of drainage network without the use of threshold. Resultant stream order raster, termed as raw stream order raster (RSOR), is tested against threshold defined stream order raster to evaluate its efficiency. Use of RSOR allows extraction of stream heads to greater stream head extent. Again, DEM downscaling takes care of overestimation in number of streams. So, the proposed technique is effective in controlling two basic aspects of drainage characteristics – stream number and extent. In our case, drainage estimated from re-sampled medium-scale DEM has the closest matching with that of the reference topographical map.     

基于OLI数据与决策树法的去山体阴影水体信息提取研究

研究山区地表水体信息OLI遥感数据去阴影自动提取方法,设计基于数字高程模型与指数提取的决策树分类方法,提高水体自动识别的精度。该方法选取改进的归一化水体指数、归一化植被指数、比值植被指数、主成分分析前3个分量以及波段之间的组合运算,并结合DEM构建决策树分类规则。综合采用单波段阈值、谱间关系、植被指数和水体指数阈值完成山体水体的去阴影识别研究,与计算机自动识别分类方法比较,其精度明显提高。结果表明,决策树分类方法在精度上明显高于常用的计算机自动分类方法,可以很好地被利用于OLI遥感数据水体信息的海量、大范围提取。     

Automatic and improved radiometric correction of Landsat imagery using reference values from MODIS surface reflectance images

Radiometric correction is a prerequisite for generating high-quality scientific data, making it possible to discriminate between product artefacts and real changes in Earth processes as well as accurately produce land cover maps and detect changes. This work contributes to the automatic generation of surface reflectance products for Landsat satellite series. Surface reflectances are generated by a new approach developed from a previous simplified radiometric (atmospheric + topographic) correction model. The proposed model keeps the core of the old model (incidence angles and cast-shadows through a digital elevation model [DEM], Earth–Sun distance, etc.) and adds new characteristics to enhance and automatize ground reflectance retrieval. The new model includes the following new features: (1) A fitting model based on reference values from pseudoinvariant areas that have been automatically extracted from existing reflectance products (Terra MODIS MOD09GA) that were selected also automatically by applying quality criteria that include a geostatistical pattern model. This guarantees the consistency of the internal and external series, making it unnecessary to provide extra atmospheric data for the acquisition date and time, dark objects or dense vegetation. (2) A spatial model for atmospheric optical depth that uses detailed DEM and MODTRAN simulations. (3) It is designed so that large time-series of images can be processed automatically to produce consistent Landsat surface reflectance time-series. (4) The approach can handle most images, acquired now or in the past, regardless of the processing system, with the exception of those with extremely high cloud coverage. The new methodology has been successfully applied to a series of near 300 images of the same area including MSS, TM and ETM+ imagery as well as to different formats and processing systems (LPGS and NLAPS from the USGS; CEOS from ESA) for different degrees of cloud coverage (up to 60%) and SLC-off. Reflectance products have been validated with some example applications: time series robustness (for a pixel in a pseudoinvariant area, deviations are only 1.04% on average along the series), spectral signatures generation (visually coherent with the MODIS ones, but more similar between dates), and classification (up to 4 percent points better than those obtained with the original manual method or the CDR products). In conclusion, this new approach, that could also be applied to other sensors with similar band configurations, offers a fully automatic and reasonably good procedure for the new era of long time-series of spatially detailed global remote sensing data.     

DEM技术在景观工程道路选线中的应用

数字高程模型(DEM)既是二维地理空间上具有连续变化特征的地理现象的模型化表达和过程模拟,又是空间起伏连续变化现象的数字化表示和分析工具的集合。道路工程是DEM应用最早的领域。在道路勘测设计中,通过高精度DEM,可以进行平面、纵断面的交互定线,横断面的自动生成、土方计算与调配、道路的多方案比选,并通过设计表面模型和DEM叠加,实现道路景观模型及动画演示,从而对设计质量进行评价,并对拟建道路与周围环境的协调状况进行分析。     

基于不同大小窗口的移动曲面拟合法探测不规则DEM粗差的一种方法

粗差的存在会造成数字高程模型(DEM)空间上的严重扭曲,有时能导致DEM及其产品严重失真,甚至完全不能使用.因而有关DEM的粗差诊断问题已愈来愈引起人们的关注.本文在对现有的基于点方式的DEM粗差探测算法进行分析的基础上,提出一种基于不同大小窗口的移动曲面拟合法探测DEM粗差的一种方法(简称YXY算法),并通过蒙特卡洛仿真试验来验证该算法的有效性和可行性.     

Improved Priority‐Flood method for depression filling by redundant calculation optimization in local micro‐relief areas

Depression filling is a critical step in distributed hydrological modeling using digital elevation models (DEMs). The traditional Priority‐Flood (PF) approach is widely used due to its relatively high efficiency when dealing with a small‐sized DEM. However, it seems inadequate and inefficient when dealing with large high‐resolution DEMs. In this work, we examined the relationship between the PF algorithm calculation process and the topographical characteristics of depressions, and found significant redundant calculations in the local micro‐relief areas in the conventional PF algorithm. As such calculations require more time when dealing with large DEMs, we thus propose a new variant of the PF algorithm, wherein redundant points and calculations are recognized and eliminated based on the local micro‐relief water‐flow characteristics of the depression‐filling process. In addition, depressions and flatlands were optimally processed by a quick queue to improve the efficiency of the process. The proposed method was applied and validated in eight case areas using the Shuttle Radar Topography Mission digital elevation model (SRTM‐DEM) with 1 arc‐second resolution. These selected areas have different data sizes. A comparative analysis among the proposed method, the Wang and Liu‐based PF, the improved Barnes‐based PF, the improved Zhou‐based PF, and the Planchon and Darboux (P&D) algorithms was conducted to evaluate the accuracy and efficiency of the proposed algorithm. The results showed that the proposed algorithm is 43.2% (maximum) faster than Wang and Liu's variant of the PF method, with an average of 31.8%. In addition, the proposed algorithm achieved similar performance to the improved Zhou‐based PF algorithm, though our algorithm has the advantage of being simpler. The optimal strategies using the proposed algorithm can be employed in various landforms with high efficiency. The proposed method can also achieve good depression filling, even with large amounts of DEM data.     

Variation of Snowline and Mass Balance of Glaciers of Warwan and Bhut Basins of Western Himalaya Using Remote Sensing Technique

Glaciers are natural reservoirs of fresh water in frozen state and sensitive indicators of climate change. Among all the mountainous glaciated regions, glaciers of Himalayas form one of the largest concentrations of ice outside the Polar Regions. Almost all the major rivers of northern India originate from these glaciers and sustain perennial flow. Therefore, in view of the importance and role of the glaciers in sustaining the life on the Earth, monitoring the health of glaciers is necessary. Glacier??s health is monitored in two ways (i) by mapping the change in extent of glaciers (ii) by finding variation in the annual mass balance. This paper has been discussed the later approach for monitoring the health of glaciers of Warwan and Bhut basins. Mass balance of glaciers of these two basins was determined based on the extraction of snow line at the end of ablation season. A series of satellite images of AWiFS sensor were analysed for extraction of snowline on the glaciers for the period of 2005, 2006 and 2007. The snow line at the end of ablation season is used to compute accumulation area ratio (AAR = Accumulation area/Glacier area) for each glacier of basins. An approach based on relationship of AAR to specific mass balance (computed in field) for glaciers of Basapa basin was employed in the present study. Mean of specific mass balance of individual glacier for the year 2005, 2006 and 2007 of Warwan basin was found to be ?ve 0.19?m, ?ve 0.27?m and ?ve 0.2?m respectively. It is 0.05?m, ?ve 0.11?m and ?ve 0.19?m for Bhut basin. The analysis suggests a loss of 4.3 and 0.83?km3 of glacier in the monitoring period of 3?years for Warwan and Bhut basins respectively. The overall results suggest that the glaciers of Warwan basin and Bhut basins have suffered more loss of ice than gain in the monitoring period of 3?years.     

基于Kalman滤波的InSAR基线估计方法

针对目前SAR干涉测量中基线估计现存的问题,提出了利用Kalman滤波和配准参数进行基线估计的方法.所提出的方法具有不需地面控制点、不受地形限制和不依赖于轨道参数等优点,并可以估计时变的基线参数.利用南京地区的ERS-1/2 tandem数据进行了试验研究,并对提出的方法进行了验证.结果表明,在精确的卫星轨道数据和地面控制点不能获取时,所提出的方法仍能有效地估计InSAR基线.这在一定程度上补偿了轨道偏移带来的误差,为获取高精度的DEM奠定了基础.     

基于数字高程模型的混合流向算法

从数字高程模型提取的汇水网络和汇水区等信息是分布式水文模型及应用分析的基础参数,基于地表汇水模拟的算法是提取该类信息的主要方法,其中,水流方向的确定对提取结果有着直接的影响。单流向算法因其易于实现、易于确定上游汇水区等特性,得到了广泛应用,然而单流向算法在坡度平缓区域会产生不自然的平行径流,能模拟地表水流分散径流特点的多流向算法可以在一定程度上避免此问题,但多流向算法使得不同区域的汇水单元可能存在交叉。本文结合两类流向算法各自的优点和适用性,设计实现了一种混合流向算法,以期在不同的地形条件下模拟得到更加合理的水流分配。首先,使用基于模板的形态检测方法,在给定阈值的基础上,对数字地形进行了分类,DEM被划分为山谷、山脊、鞍部、缓坡和陡坡5类。对陡坡、山谷和山脊区域运用单流向算法;对缓坡和鞍部区域采用多流向算法确定径流方向并进行水量分配。本文选取了黄土地貌和中低山丘陵的两个流域作为研究区,利用并采用了30 m和90 m两个分辨率的DEM。本文研究将混合流向算法与现有其他算法的结果进行比较。相比于多流向算法,该算法结果中的分散效应受到明显的抑制,相比于单流向算法,非自然的平行径流也大幅减少。同时,混合流向算法在较大分辨率DEM上（30 m）改进效果更加明显。     

Phenomenon-based Specification of the Digital Representation of Terrain Surfaces

The phenomenon known as 'terrain' is a continuous surface. However, when a digital terrain representation is based on a regular raster (i.e. a DEM) the digital surface is commonly not continuous. This is the case for the derivation of variables such as slope, aspect, and curvature values as performed in today's Geographic Information Systems (GIS). Often, there is no surface specified at all, as, for instance, when flow lines or watersheds are constructed. The discrepancy between the phenomenon to be modelled and its digital representation causes the terrain analysis results to be less accurate than they could be. Furthermore, if more than one type of terrain information is derived the results are likely to be based on different specifications of the seemingly same terrain surface. The combined application of the derivation results will likely introduce inconsistencies. This paper suggests founding the specification of digital terrain representations on a careful analysis of the properties of the phenomenon. The paper details the reasons for, and advantages of, continuous surface representations and emphasises the importance of a comprehensive documentation of the conceptual models underlying digital terrain representations. A review of suitable interpolation approaches for the specification of terrain surfaces is given. The paper discusses how the resulting digital surfaces are analysed and how measurement uncertainty may be accounted for.     

基于粗DEM的立体SAR像对配准

影像配准是雷达立体摄影测量技术获取数字高程模型的一个关键步骤,也是一大难点。为了能够自动而又稳健地获取高精度的立体SAR像对配准结果,文中设计一种基于粗DEM、距离多普勒SAR构像模型和基于区域逐级窗口的立体SAR像对配准方法。该方法采用距离多普勒SAR成像模型和粗DEM获取像素级的粗略配准结果。在此基础上,采用基于区域的逐级窗口配准方法获取高精度的配准结果。文中最后利用立体SAR像对进行实验研究。实验结果表明,设计的立体像对配准方法可行,其配准精度可以达到子像素级,距离向为0.3像素左右,方位向为0.1像素左右,完全能够满足雷达立体测图的要求,而且该方法还具有很好的稳健性。     

Rational function model-based image matching for digital elevation models

This paper presents an image matching technique for IKONOS satellite imagery based on rational function models (RFMs). This algorithm adopts the object-space approach and reduces the search space to within the confined line-shaped area called the piecewise matching line (PML). Also, the detailed procedure of generating 3D surface information using the RFM coefficients (RFCs) is introduced to illustrate an end-user's point of view. The final digital elevation model (DEM) generated using the proposed scheme shows a mean error of 2·2 m and rmse of 3·8 m compared with that from a 1:5000 scale digital map.