Distinguishing actual and artefact depressions in digital elevation data

Topographic depressions in digital elevation models (DEMs) are frequently a combination of artefacts and actual features. It is common practice to remove all digital depressions, from DEMs that are used in hydrogeomorphic applications. This practice is inappropriate because actual depressions affect many of the environmental phenomena at study. Nonetheless, indiscriminate depression removal persists because of an inability to distinguish artefacts from actual depressions.Five potential approaches for distinguishing artefacts from actual depressions in DEMs are described in this paper: ground inspection, examining the source data, classification approaches, knowledge-based approaches, and modelling approaches. Of the five methods, ground inspection was the only approach that actually confirms the existence of digital depressions. The other four methods that were identified operate by establishing justification for why a digital depression is likely to be an artefact or actual depression. A comparison of the depression validation approaches for a small sub-catchment on the Canadian Shield showed that the modelling approach performed slightly better than the other methods. While being highly automated and applicable to all landscape types, this approach also explicitly handles DEM uncertainty. By applying the Monte Carlo method, this approach estimates the likelihood of a digital depression actually occurring in the landscape given the degree of uncertainty in local topography. After artefact and actual depressions are identified, it is then possible to remove the artefacts and to preserve the real features for incorporation into modelling.     

Digital elevation model generation using ascending and descending ERS-1/ERS-2 tandem data

In this study we show how ascending and descending ERS-1/ERS-2 tandem data can be used to generate a precise digital elevation model (DEM). In particular we present the advantages, for DEM generation, of combining ascending and descending interferometric data pairs with large baselines; unwrapping problems caused by the high fringe rates of these data are minimized by applying a flattening technique based on the use of a data pair with smaller baseline. The presented experiments carried out on the data of an area with steep topography (Etna volcano, Italy) confirm the potentiality of the proposed technique.     

基于无人机影像的山地人工林景观DEM构建

山地人工林景观的DEM构建是对景观地形信息进行描述的基础的研究内容,也是人工林景观面积、结构、蓄积量等信息提取的重要因子,具有重要的研究意义。通过无人机平台获取影像,采用立体像对拼接的方式生成正射影像并提取DEM信息,并与GPS测量数据、ASTER GDEM、SRTM数据进行比较分析。结果表明:在该区域无人机影像构建的DEM与实测高程差距最小(RMSE=8.96),具有比ASTER GDEM(RMSE=13.68)和SRTM(RMSE=11.81)更高的精度;在每个样方内的最大高程差值与最大树高最为接近(RMSE=1.813),说明无人机DEM能够反映出更多的冠层与地面分层信息,在山地人工林景观DEM构建中表现出较大潜力。     

Waterline mapping at the subpixel scale from remote sensing imagery with high‐resolution digital elevation models

Subpixel mapping technology is a promising method of increasing the spatial resolution of the classification results derived from remote sensing imagery. However, for waterline mapping problems, the traditional spatial dependence principle of subpixel mapping is not suitable as the water flow is always controlled by the topography. This letter presents a novel algorithm based on a high spatial resolution digital elevation model (DEM) to address the subpixel waterline mapping problem. The waterline was mapped at the subpixel scale with a proposed rule according to the physical features of the water flow and additional information provided by the DEM. The method was evaluated with degraded real remotely sensed imagery at different spatial resolutions. The results show that the proposed method can provide more accurate classifications than the traditional subpixel mapping method. Moreover, the fine spatial resolution DEM can be used as feasible supplementary data for subpixel waterline mapping from coarser spatial resolution imagery.     

A Radiant Topographical Correction Model of Remote SensingImageries by Considering Micro-terrain

The radiant topographical correction is an important preprocessing for the interpretation of remote sensing imageries.A radiant topographical correction model is proposed with high-resolution DEM data by considering micro-terrain.The model simulates the effects of the micro-terrain units on pixels’ radiance.These micro-terrain units in a single pixel are used to describe topographical non-Lambertian reflectance characteristics based on V-slope illumination model.The experiments using several images have proved that the results of model that was applied in this study are better than that of the conventional correction methods.     

Modeling the effects of elevation data resolution on the performance of topography-based watershed runoff simulation

The spatial uncertainty of a topography based rainfall runoff model (TOPMODEL) is addressed in this study to assess its variability in simulating watershed hydrologic response with regards to the change of digital elevation model (DEM) resolution. Twelve DEM realizations of different grid sizes ranging from 30 m to 3000 m for each of two case watersheds are used for comparative examinations. The study shows that DEM grid size has significant influence on the topographic index distribution which represents the effect of topography on watershed hydrology in TOPMODEL. The smoothing effect of grid size increase may result in deteriorated topographic index distributions at coarse resolutions as the ratio of grid cell area to watershed area gets larger. The simulated discharges and model efficiencies using a same set of TOPMODEL parameters are sensitive to DEM grid size especially at coarse resolutions. This sensitivity, however, can be moderated by parameter calibrations as the optimization runs show that fairly equal efficiencies can be preserved by the compensation effect of transmissivity parameter T₀ within a large extent of DEM resolution for each watershed. The interaction between T₀ and the topographic index distribution with respect to TOPMDOEL model performance is also examined. It is found that both study watersheds demonstrate a similar pattern of change in model performance along with the increase of the grid-to-watershed ratio. The analysis reveals that the ratio poses an important factor in controlling the effect of DEM grid size on TOPMODEL performance. A ratio of less than 5% is suggested in DEM resolution selection for TOPMODEL applications based on the results of this study.     

Extracting digital elevation models from SAR data through independent component analysis

This article illustrates findings about a new method for the extraction of digital elevation model (DEM) from interferograms of synthetic aperture radar (SAR) data, based on independent component analysis (ICA). This method is named InSAR-ICA, and its advantages are related to its capability in estimating topographical altitudes without taking into account any or sparse non-SAR data (e.g. ground-based or atmospheric parameters). This is especially useful in cases of mountainous regions, where large altitude changes over short distances and the presence of microclimates generally limit the possibility of DEM estimates by few SAR interferograms only. A comparison between the standard SRTM (Shuttle Radar Topographical Mission) elevations and those extracted by InSAR-ICA is carried out for the region of South Italy, Apennine Mountains. Results are interpreted in terms of the reliability of InSAR-ICA DEM estimates.     

VR-GIS技术在数字黑河流域飞行模拟中的应用

以黑河流域为例,将VR-GIS技术应用在数字流域的飞行模拟中。资料预处理阶段,制作了流域Landsat TM/ETM+数字镶嵌图、DEM数据和矢量图层。利用ERDAS IMAGINE的VisualGIS模块,集成多层数据,通过人机交互进行参数设定,最终实现流域的飞行模拟。飞行模拟结果表明,从黑河上游到下游动态地俯视其沿线的地形、地貌、植被覆盖状况,浏览者可以获得比二维数字镶嵌图更为直观、生动和形象的认识。     

Effect of SRTM resolution on morphometric feature identification using neural network—self organizing map

In this study, we present a semi-automatic procedure using Neural Networks—Self Organizing Map—and Shuttle Radar Topography Mission DEMs to characterize morphometric features of the landscape in the Man and Biosphere Reserve “Eastern Carpathians”. We investigate specially the effect of two resolutions, SIR-C with 3 arc seconds and X-SAR with 1 arc second for morphometric feature identification. Specifically we investigate how the SRTM/C band data with 30 m interpolated grid, corresponding to SRTM/X band 30 m, affect the morphometric characterization and topography derivatives. To reduce misregistration between the DEMs, spatial co-registration was performed and a RMSE of 0.48 pixel was achieved. Morphometric parameters such as slope, maximum curvature, minimum curvature and cross-sectional curvature are derived using a bivariate quadratic approximation on 90 m, 30 m and interpolated 30 m DEMs. Self Organizing Map (SOM) is used for the classification of morphometric parameters into ten exclusive and exhaustive classes. These classes were analyzed as morphometric features such as ridge, channel, crest line and planar for all data sets based on feature space (scatter plot), morphometric signatures and 3D inspection of the area. The map quality is analyzed by oblique views with contour lines overlaid. Using the X band DEM with 30 m grid as benchmark, a change detection technique was used to quantify differences in morphometric features and to assess the scale effect going from a 90 m (C-band) DEM to an interpolated 30 m DEM. The same procedure is used to study the effect of different resolutions on morphometric features. Morphometric parameters were computed by a moving window size 5 × 5 (corresponding to 450 m on the ground) over SRTM- 90 m. To cover the same ground area, a moving window size of 15 × 15 is used for the 30 m DEM. The change analysis showed the amount of resolution dependency of morphometric features. Overall, the results showed that the introduced method is very useful for identification of morphometric features based on SRTM resolution. Decreasing the grid size from 90 m to 30 m reveals considerably more detailed information emphasizing local conditions. Comparison between results from DEM-30 m as reference data set and interpolated 30 m, showed a rate of change of 31.5% which is negligible. About 17% of this rate correspond to classes with mean slope > 10°. Of the morphometric parameters, the cross sectional curvature is most sensitive to DEM resolution. Increasing spatial resolution reduces the main constrains for morphometric analysis with SRTM 90 m data, such as unrealistic features and isolated single elements in the output map. So in case of lack of high resolution data, the SRTM 90 m data could be interpolated and used for further geomorphic analysis.     

Delineating dambo catenary soil-landscape units using aerial gamma-ray and terrain data: a comparison of classification approaches

Soil data are largely absent for most of Africa. For landscapes with recognizable catenary elements, this data gap can be filled by mapping the catenary units and assigning them with known soil properties. An example is the landscape map for a region with dambos in central Uganda, which shows the four catenary units in order from well-drained to seasonal wetland: uplands, margins, floors, and bottoms. However, this map was created using optical data, which are cost prohibitive and are also limited by cloud cover. We evaluated the potential of freely available aerial gamma-ray spectrometry (AGRS) data as an alternative source of classification inputs. Analysis of variance based upon field data for a region with dambos in central Uganda showed gamma activity to differ along the catenary sequence, with landscape position explaining an appreciable proportion of variation of potassium (28%), thorium (27%), and uranium (46%). Using the three gamma channels, together with terrain indices derived from the Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) as inputs, three classifiers were evaluated – conditional inference trees (CITs), random forests (RF), and multinomial-iterative self-organizing data analysis (ISODATA). While untransformed terrain and gamma predictors were used for the first two methods, we applied the ISODATA classification to landscape unit probability maps generated using multinomial principal components regression. For the CIT classification, all decision rules were based on terrain data, which might explain why the map was slightly less accurate (unweighted kappa = 0.61, linear weighted kappa = 0.73) than the map created using a RF classifier (unweighted kappa = 0.63, linear weighted kappa = 0.74), where both terrain and gamma predictors were used. But the existence of artefacts of margins within uplands in the map based on CIT modelling, and not that created using RF, is because the former missed the smoothing effect of gamma, attributed to zonal differences in activity of all three gamma channels. The multinomial-ISODATA predictions were poor (unweighted kappa = 0.56, linear weighted kappa = 0.67), partly because the regression model could not adequately resolve differences between bottoms and floors. However, we did find the probability maps generated using multinomial regression to be useful end products that capture the continuous nature of landscape unit transitions. It is important to note that in this study we used 90 m grid resolution gamma and terrain data to predict features that transition over distances of less than 10 m, so better results might be possible with finer-resolution gamma and terrain data.     

Application of high-resolution, interferometric DEMs to geomorphic studies of fault scarps, Fish Lake Valley, Nevada-California, USA

Recent developments in interferometric radar remote sensing provide a method for deriving detailed topographic and slope information for fault-scarp detection. In October 1996, the airborne TOPSAR instrument [IEEE Trans. Geosci. Remote Sens., 30 (1992) 933] was flown over southwest Nevada and east-central California. Topography calculated from TOPSAR data are in the form of a high-resolution (5-m spatial grid) digital elevation model (DEM). In this study, we focus upon the large, steep fault scarps that cut alluvium and alluvial fans in Fish Lake Valley, east of the White Mountains of Nevada-California. A series of topographic profiles extracted from the DEM reveal that the larger fault scarps are greater than 40 m in height and that the average midsection slope angle for all measured scarps is approximately 23°. These large scarps are the product of multiple offsets rather than a single event. Other relevant geomorphic features present in the digital topography include splays and benches along the main fault, levees, cutbanks, gullies incising fault scarp slopes, shutterridges, offset drainage, and small normal faults with scarp heights of only 4-6 m. Field work corroborated general geomorphologic landforms, confirmed fault-scarp morphometry, and aided the evaluation of the accuracy of the DEM. We are also able to assess fault segmentation models that divide the Fish Lake Valley fault zone (FLVFZ) into discrete segments based upon surface-rupture characteristics.     

Relation between landform and vegetation in alpine regions of Wallis,Switzerland. A multiscale remote sensing and GIS approach

The analysis of habitat factors for the distribution of vegetation based on the analysis of landform characteristics is an important aspect in the process of understanding high mountain ecology. Therefore in the present study a GIS and remote sensing based approach is followed to produce different scale vegetation maps for a study area in the Western Alps (Switzerland). As spatial information on site factors is commonly lacking in mountain areas, the use of a Digital Elevation Model (DEM) is a potential substitute for use in vegetation analyses, as it highly correlates with temperature, moisture, geomorphological processes and disturbance factors. Thus it is crucial to analyse the capabilities of a DEM for indicating habitat conditions in a landscape characterised by high topodiversity and a patchwork of microclimatic habitats. Appropriate landform parameters have been derived, indicating temperature and moisture distribution, exposure towards wind, snow etc. Using contingency tables and principal components analysis the overall influence of topography and landform on vegetation distribution was analysed. Nevertheless, the lack of information on the human dimension leads to some uncertainties in the interpretation of spatial patterns of vegetation. Additionally landform classification schemes decomposing the landscape into basic landform-elements only proved useful for characterising azonal, non-altitudinal vegetation classes.     

多尺度小波分析用于DEM网格数据综合

数字高程模型(DEM)虽已广泛应用于地学、景观以及区域规划等工作，然而在不同尺度下，不同的系统对其数据量的要求是不同的，且数据量将影响系统运行及分析的速度；其次等高线的制图综合也可直接通过数字高程模型数据的综合来实现，为了加快DEM数据综合的速度和保持综合前后DEM数据特征一致性，提出了一种采用小波多尺度分析的方法来进行DEM网格数据综合的方案，并给出了数字高程模型网格数据的综合基本模型。同时对综合以后的可靠性分别从数据量、断面图和曲面面积变化等方面进行了分析。具体实例说明，将多尺度小波分析用于DEM网格数据综合是有效的。     

Technical note: An evaluation of interpolation methods for Mars Orbiter Laser Altimeter (MOLA) data

The Mars Orbiter Laser Altimeter (MOLA) instrument on the Mars Global Surveyor (MGS) spacecraft has returned a large amount of data on the topography of Mars. It is possible to generate high resolution digital elevation models (DEMs) from these data by employing data interpolation techniques. Four interpolation algorithms were selected for testing on MOLA data: Delaunay-based linear interpolation, splining, nearest neighbour, and natural neighbour. These methods were applied to the MOLA data of Korolev crater for qualitative analysis. In addition, a DEM of a part of Iceland was used for quantitative testing by simulating MOLA data acquisition, interpolating those data, and then calculating the mean absolute error (MAE) between the interpolated and original DEM. Execution speeds were measured for the four algorithms. The natural neighbour method proved superior both quantitatively and qualitatively to other methods tested, but is relatively slow computationally.     

Using satellite remote sensing for DEM extraction in complex mountainous terrain: Landscape analysis of the Makalu Barun National Park of eastern Nepal

The design and management of national parks and other protected areas requires a broad base of physiographic and geo-ecological information about the landscape. This paper evaluates the effectiveness of satellite remote sensing for photogrammetric stereo-mapping and digital elevation model (DEM) extraction within remote mountainous terrain. As a case study, a landscape analysis of the Makalu Barun National Park and Conservation Area of east Nepal (27.5° N, 87.0° E) was examined. The study area is a highly complex and rugged mountain landscape, with extreme topographic relief and an elevation gradient spanning more than 8300 m. A DEM extracted from stereo SPOT imagery resulted in a median disagreement of 58 m when compared to a DEM generated from a conventionally digitized GIS dataset of topographic contours (scale=1:250 000). Visual comparison of the two DEMs showed substantial agreement at the landscape scale, while larger scale comparison of 100 m contours revealed some localized differences. The SPOT extracted DEM provided equal or better basis for orthorectification of satellite imagery when compared to the conventional DEM. Derivative landscape analysis outputs, such as hydrological modelling, drainage networks and watershed boundaries, compared well with results based upon the conventional dataset. Intermediate map products useful for field research and mapping included production of an orthorectified satellite base-map image. Additionally, a fused multisensor high resolution image of the study area, combining Landsat Thematic Mapper (TM) and SPOT imagery at 10 m resolution, was orthorectified to produce a false-colour satellite image map highlighting the spectral discrimination between land cover classes.     

Impact of DEM accuracy and resolution on topographic indices

Topography is an important land-surface characteristic that affects most aspects of the water balance in a catchment, including the generation of surface and sub-surface runoff; the flow paths followed by water as it moves down and through hillslopes and the rate of water movement. All of the spatially explicit fully distributed hydraulic and hydrological models use topography (represented by the DEM of the area modelled) to derive bathymetry. DEM is also used to derive some other key information critical in fully distributed hydraulic and hydrological models.With high-resolution DEMs such as LiDAR (Light Detection and Ranging) becoming more readily available and also with the advancements in computing facilities which can handle these large data sets, there is a need to quantify the impact of using different resolution DEMs (e.g. 1 m against 10 m or 25 m) on hydrologically important variables and the loss of accuracy and reliability of the results as we move from high resolution to coarser resolution.The results from statistical analysis carried out to compare field survey elevations with the LiDAR DEM-derived elevations, show that there are small differences between the two data sets but LiDAR DEM is a reasonably good representation of the actual ground surface compared to other commonly used DEMs derived from contour maps.The results from the analysis clearly show that the accuracy and resolution of the input DEM have serious implications on the values of the hydrologically important spatial indices derived from the DEM. The result also indicates that the loss of details by re-sampling the higher resolution DEM to coarser resolution are much less compared to the details captured in the commonly available coarse resolution DEM derived from contour maps. Topographic indices based on contour derived DEMs should be used with caution and where available, the higher resolution DEM should be used instead of the coarse resolution one.     

Technical Note: DEM from IRS-1C PAN stereo coverages over Himalayan glaciated region—accuracy and its utility

Satellite photogrammetric technique has been used for monitoring of fluctuations of Himalayan glaciers and the resulting changes in the elevations of glacier snouts. Two across-track stereo pairs from the Indian Remote Sensing Satellite (IRS)-1C covering parts of Basapa valley, a high altitude Himalayan glaciated terrain, were processed for generation of digital elevation models (DEM) and orthoimages using a softcopy photogrammetry workstation. Two glacier regions, viz., Janapa Garang and Shaune Garang glacier valley, were taken up for the study. For one glacier region, the stereo pair was generated in a workstation by replacing an image of one pair with an image from the other pair. Interchanging input image and reference image in the new pair has resulted in improvement in image matching. Accuracies related to location (in terms of latitude and longitude) and elevation of image features in the accumulation zone, ablation zone and nonglaciated regions with respect topographical map were checked. Based on geographical location and elevation of the snout derived from a topographical map of 1962 and the DEM and orthoimage of 1997, the two glaciers have been observed to have a retreat of about 690?m and 925?m, respectively.     

一种新的山脊线和山谷线自动提取方法

提出了一种新的自动提取山脊线和山谷线的方法－方向剖面法。该方法可以根据穿过高程数据点的各个方向的剖面线特征,自动识别DEM中可能的山脊点和山谷点;然后,用修正的Hilditch细线化算法筛选山脊点和山谷点;接着再把筛选后两两相邻的山脊点或山谷点连接起来,形成山脊线或山谷线的“雏形”;最后,将山脊线或山谷线“雏形”中封闭的三角形断开,并剔除过短的山脊线和山谷线。用真实DEM数据进行试验研究,其自动提取的山脊线和山谷线与高程等值线图反映的地形起伏基本一致,从而证明了该方法的有效性。     

Inertial thin film flow on planar surfaces featuring topography

A range of problems is investigated, involving the gravity-driven inertial flow of a thin viscous liquid film over an inclined planar surface containing topographical features, modelled via a depth-averaged form of the governing unsteady Navier-Stokes equations. The discrete analogue of the resulting coupled equation set, employing a staggered mesh arrangement for the dependent variables, is solved accurately using an efficient full approximation storage (FAS) algorithm and a full multigrid (FMG) technique; together with error-controlled automatic adaptive time-stepping and proper treatment of the associated nonlinear convective terms. An extensive set of results is presented for flow over both one- and two-dimensional topographical features, and errors quantified via detailed comparisons drawn with complementary experimental data and predictions from finite element analyses where they exist. In the case of one-dimensional (spanwise) topography, moderate Reynolds numbers and shallow/short topographical features, the results obtained are in close agreement with corresponding finite element solutions of the full free-surface problem. For the case of flow over two-dimensional (localised) topography, it is shown that the free-surface disturbance is influenced significantly by the presence of inertia leading, as in the case of spanwise topography, to an increase in the magnitude and severity of the resulting capillary ridge and trough formations: the effect of inclination angle and topography aspect ratio are similarly explored.     

机载AISA Eagle Ⅱ高光谱数据处理——以额济纳旗试验区为例

机载AISA EagleⅡ传感器为"黑河综合遥感联合试验(HiWATER)"额济纳旗试验区提供航空高光谱影像。介绍了高光谱原始数据的辐射定标、几何校正、大气校正等预处理过程。根据研究区地形差异以及数据使用目的的多样性,几何校正中可选择是否加高精度DEM产品,大气校正的选择策略可分为平坦地形无DEM的大气校正和起伏地形添加DEM大气校正。本试验数据采用加载高精度DEM的几何校正和平坦地形大气校正方法,经过预处理后的高光谱数据产品,其地理坐标与高分辨率的CCD影像对比,地理位置信息较为准确;与实测地物光谱对比,影像光谱能较好地体现地物光谱的特性,数据可用作定量遥感进一步的研究。