DEM结构特征对坡度坡向的影响分析

数字高程模型已严格定义为按规则格网阵列记录的地形高程数据,其固有的结构特征(如格网分辨率、格网方向、高程数据准确度等)直接影响DEM对地形表达和坡度、坡向的计算精度。该文通过理论和数据独立的DEM实验分析方法,研究了DEM结构特征对坡度、坡向的影响,得出如下结论:1)高分辨率的DEM并不一定能给出高精度的坡度、坡向计算结果;2)可通过g=bm/ms×180/π×cos2S来选择合适的DEM分辨率;3)三阶不带权差分算法的坡度、坡向计算结果对DEM方向有较强的依赖性。     

地形复杂度对坡度坡向的影响

采用三阶不带权差分算法，研究了地形复杂度与坡度坡向的关系，澄清了目前关于坡度坡向误差空间分布的矛盾观点，并分别在凹向椭球和高斯合成曲面数学模型曲面DEM上对其进行验证。通过研究得出：①坡度、坡向误差与坡度值正相关；②坡度坡向误差主要分布在平坦地区；③坡向误差较坡度误差对DEM高程数据误差敏感，较小的DEM误差引起较大的坡向误差。     

基于DEM的任意方向坡度计算方法

坡度是数字地形分析中重要的地形因子,在水文分析、土壤侵蚀模拟、地貌类型划分等地学分析及工程上均有广泛的应用。由于地形的各向异性,坡度在各个方向上并不相同。目前,基于DEM所提取的坡度,均是位于最陡方向上的坡度,而任意给定方向坡度计算是地学分析和工程应用中较为常见的问题。在对目前DEM坡度提取算法研究的基础上,通过数学分析,给出了格网DEM上任意方向格网点的坡度计算模型和计算流程,并给出了相应的计算实例,从而将坡度计算模型从特定方向推向任意方向,使DEM坡度计算更具普适性。     

坡度的尺度效应及其对径流模拟的影响研究

研究不同尺度的数字高程模型所带来的坡度差异对水文模型径流模拟的影响.从坡度的尺度效应出发,讨论了相同DEM条件下不同格网大小造成的坡度差异,通过模拟研究发现,随着格网的增大,流域平均坡度在整体上虽然呈减小的趋势,但在不同的格网范围,流域平均坡度的变化趋势并不一致,对平均坡度与不同阶段变化的DEM格网大小采用不同的曲线进行拟合后发现,在某个范围平均坡度的变化比较缓慢,进而可以得到研究区水文模型最佳的DEM格网大小.研究表明,合适的DEM尺度对于水文模型的研究和应用具有重要作用.     

基于不同方向参考系DEM提取坡度的差异性分析

坡度指标通常基于DEM数据以固定算法直接提取。坡度误差主要来源于算法模型误差和数据误差等,很少顾及在建立DEM时空间参考系方向的变化引起的坡度差异。本文以数学高斯曲面为基准,通过改变参考系X、Y轴方向,对不同参考系下的DEM数据以三阶反距离平方权差分坡度算法提取坡度并分析其差异,结果表明:1.空间参考系方向变化对坡度差异的影响与地表剖面曲率有关,在山顶、谷底以及鞍部等坡度变化较为明显的地形区域,较为显著,两者成一定的正相关,而且与坡向变化率也存在相关关系。2.坡度、坡向差异随着空间参考系方向变化呈现周期性,周期为90°,近似按正弦(y=a·sin(1/2kπ+φ)φ∈[0,π/2])规律变化,在45°处达到峰值,而在0°与90°附近,6°范围内平均差异变化较为平缓,但与正弦曲线偏离较大,且随着空间分辨率的降低,参考系方向引起的坡度、坡向差异有增加的趋势。实验表明在研究区建立独立参考系时应顺应平均坡向原则,以减小参考系方向对坡度、坡向的影响。     

基于矩匹配算法的山区影像地形辐射校正方法研究

针对山区遥感影像地形辐射校正问题,应用现有模型(如C校正)进行地形辐射校正很难达到理想效果。引入矩匹配算法,利用DEM数据计算坡度、坡向等地形信息,以特定坡度和坡向数据为参考依据,对影像进行地形辐射校正。通过北京房山区SPOT5影像进行试验,表明该方法能在很大程度上消除地形阴影,更好地反映阴影区域的细节信息,同时光谱特性保真程度较好,原模糊的影像区域通过处理基本上达到有效识别地物的要求。     

面栅格表达方式引起的DEM地形表达误差

通过对规则格网DEM对地面表达方式的分析,指出基于面栅格理解的DEM的表达方式会产生DEM对实际地形的表达误差,该误差与格网尺寸密切相关。在一个DEM格网内,地形表达误差分为正负两个部分,在上坡方向,误差为负值,下坡方向则为正值;格网尺寸越大,误差的其空间分布越复杂;在地面坡度较大时,距离格网中心点越远,误差越大。通过基于全数字摄影测量得到的黄土高原区陕西省绥德县多种格网尺寸DEM数据系列的比较验证,结果表明,在格网内部的误差分布随格网尺寸密切相关,特别是在地形复杂的地区,误差的绝对值较大,且空间分布越加复杂。根据对该误差的统计特征及中误差与DEM格网尺寸关系分析,可利用格网尺寸估计误差的统计特征及中误差。随着格网尺寸增大,中误差和误差最大值都在增大,均值基本不变;且无论基准DEM格网尺寸大小,在增大格网尺寸的情况下,均有此规律。在实际DEM生产及应用过程中,需要注意区分上述地形表达误差与我国测绘标准中定义的中误差的区别,避免在DEM的实际应用中出现偏差。     

DEM数据可视化在地震应急中的应用

数字高程模型(Digital Elevation Model简称DEM)是通过一组有序数值阵列对地面海拔高度数字描述的实体地面模型,是数字地形模型(Digital Terrain Model,简称DTM)的一个子集。并可通过对DTM数字处理得到其他各种地形特征值。DTM是描述包括高程在内的各种地貌因子,如坡度、坡向、坡度变化率等因子在内的线性和非线性组合的空间分布,其中DEM是单一的数字地貌模型,通过对DEM数据处理可派生出坡度、坡向及坡度变化率等地貌特性。基于ARCGIS技术平台,对DEM数据进行处理建模,可以得到地形真实情景再现,为震后应急指挥人员、工作人员了解震区的地貌、地形提供直接的数据支撑。     

一种基于坡度分析的DEM数字水印算法

提出一种新的基于坡度分析的DEM数字水印算法.该算法以DEM坡度为主要研究对象,由于坡度误差主要集中在平坦地区,而经小波变换后的DEM低频系数信息可反映DEM区域的地形复杂度,因此,通过对DEM低频系数进行分析,可自适应地确定水印的嵌入位置;然后以加性法则将水印信息嵌入低频系数,再经小波逆变换,即可得到含水印信息的DEM数据.实验结果表明,该算法能完全满足水印的不可见性,对DEM数据的高程精度、坡度精度及等高线的提取精度影响都很小,能够满足一定的应用要求,且具有良好的抗噪能力.     

基于DEM的长江上游土地利用分析

全球环境变化与可持续发展是当今关切的重大问题之一,而土地利用/覆盖变化研究则是其中的热点和前沿领域。目前,严重的生态环境已成为长江上游地区经济社会发展的重大障碍,也是本区和整个长江流域实现可持续发展的最主要的制约因素。本文在DEM基础上,分析了2000年长江上游高程、坡度和坡向与土地利用的关系。研究结果表明,随着高程、坡度和坡向的变化,2000年长江上游的土地利用类型也有很大差异,这些因素对长江上游的土地利用有着重要的影响。为了保护生态环境,促进本区和整个长江流域实现可持续发展,需进一步因地制宜,结合高程、坡度和坡向等影响因素优化土地利用类型和改善地表覆盖。     

Terrain complexity and uncertainties in grid‐based digital terrain analysis

The objective of this research is to study the relationship between terrain complexity and terrain analysis results from grid‐based digital elevation models (DEMs). The impact of terrain complexity represented by terrain steepness and orientation on derived parameters such as slope and aspect has been analysed. Experiments have been conducted to quantify the uncertainties created by digital terrain analysis algorithms. The test results show that (a) the RMSE of derived slope and aspect is negatively correlated with slope steepness; (b) the RMSE of derived aspect is more sensitive to terrain complexity than that of derived slope; and (c) the uncertainties in derived slope and aspect tend to be found in flatter areas, and decrease with increasing terrain complexity. The study shows that although primary surface parameters can be well defined mathematically, the implementation of those mathematical models in a GIS environment may generate considerable uncertainties related to terrain complexity. In general, when terrain is rugged with steep slopes, the uncertainty of derived parameters is quite minimal. While in flatter areas, the DEM‐based derivatives, particularly the aspect, may contain a great amount of uncertainty, causing significant limitation in applying the analytical results.     

DEM流径算法的相似性分析

流径算法是分布式水文模型、土壤侵蚀模拟等研究中的关键技术环节,决定着汇水面积、地形指数等许多重要的地形、水文参数的计算。本文以黄土高原两个典型样区的不同分辨率DEM为研究对象,对常用的五种流径算法(D8、Rho8、Dinf、MFD和DEMON)通过相对差系数、累积频率图、XY散点分布图等进行了定量的对比分析。结果表明:算法的差异主要集中在坡面区域,汇流区域各类算法的差别较小;算法差异在不同DEM尺度下都有所体现,但高分辨率下的差异会更明显;在地形复杂区域,多流向算法要优于单流向算法。研究也进一步指出汇水面积、地形指数等水文参数对流径算法具有强烈的依赖性。     

Error assessment of grid-based flow routing algorithms used in hydrological models

This paper reports an investigation on the accuracy of grid-based routing algorithms used in hydrological models. A quantitative methodology has been developed for objective and data-independent assessment of errors generated from the algorithms that extract hydrological parameters from gridded DEM. The generic approach is to use artificial surfaces that can be described by a mathematical model, thus the ‘true’ output value can be pre-determined to avoid uncertainty caused by uncontrollable data errors. Four mathematical surfaces based on an ellipsoid (representing convex slopes), an inverse ellipsoid (representing concave slopes), saddle and plane were generated and the theoretical ‘true’ value of the Specific Catchment Area (SCA) at any given point on the surfaces could be computed using mathematical inference. Based on these models, tests were made on a number of algorithms for SCA computation. The actual output values from these algorithms on the convex, concave, saddle and plane surfaces were compared with the theoretical ‘true’ values, and the errors were then analysed statistically. The strengths and weaknesses of the selected algorithms are also discussed.     

The impact of resolution on the accuracy of hydrologic data derived from DEMs

Hydrologic data derived from digital elevation models (DEM) has been regarded as an effective method in the spatial analysis of geographical information systems (GIS). However, both DEM resolution and terrain complexity has impacts on the accuracy of hydrologic derivatives. In this study, a multi-resolution and multi-relief comparative approach was used as a major methodology to investigate the accuracy of hydrologic data derived from DEMs. The experiment reveals that DEM terrain representation error affects the accuracy of DEM hydrological derivatives (drainage networks and watershed etc.). Coarser DEM resolutions can usually cause worse results. However, uncertain result commonly exists in this calculation. The derivative errors can be found closely related with DEM vertical resolution and terrain roughness. DEM vertical resolution can be found closely related with the accuracy of DEM hydrological derivatives, especially in the smooth plain area. If the mean slope is less than 4 degrees, the derived hydrologic data are usually unreliable. This result may be helpful in estimating the accuracy of the hydrologic derivatives and determining the DEM resolution that is appropriate to the accuracy requirement of a particular user. By applying a threshold value to subset the cells of a higher accumulation flow, a stream network of a specific network density can be extracted. Some very important geomorphologic characteristics, e.g., shallow and deep gullies, can be separately extracted by means of adjusting the threshold value. However, such a flow accumulationbased processing method can not correctly derive those streams that pass through the working area because it is hard to accumulate enough flow direction values to express the stream channels at the stream's entrance area. Consequently, errors will definitely occur at the stream’s entrance area. In addition, erroneous derivatives can also be found in deriving some particular rivers, e.g., perched (hanging up) rivers, anastomosing rivers and braided rivers. Therefore, more work should be done to develop and perfect the algorithms.     

The impact of resolution on the accuracy of hydrologic data derived from DEMs

1 Introduction Automated extraction of drainage features from DEMs is an effective alternative to the tedious manual mapping from topographic maps. The derived hydrologic characteristics include stream-channel networks, delineation of catchment boundaries, catchment area, catchment length, stream-channel long profiles and stream order etc. Other important characteristics of river catchments, such as the stream-channel density, stream-channel bifurcation ratios, stream-channel order, number…     

DEM地形信息量计算的不确定性研究

DEM地形信息量的准确度量是判定DEM数据应用适宜性与限制性关键。以黄土丘陵沟壑区DEM及其派生的坡度、坡向DTM为例进行实验,研究子集划分与分级数对DEM信息量估算精度的影响。结果表明,DEM地形信息量是实际地形信息和子集划分策略共同作用的结果,现有DEM信息量计算存在不确定性,DEM子集划分算法和分级数的选择直接影响DEM信息量的计算结果。文章借助最大熵定理的基本思路构建DEM地形信息量分级判定模型,可客观获得连续型栅格地形数据在信息量计算上最优分级,为有效估算DEM地形信息量提供科学依据。     

Crossing natural and data set boundaries: coastal terrain modelling in the South-West Finnish Archipelago

Geographical information systems (GIS) are important tools in coastal research and management. Coastal GIS applications involve special challenges, because the coastal environment is a complex transitional system between the terrestrial and marine realms. Also acquisition methods and responsibilities for spatial data (and thus their properties) change at the shoreline. This article explores the consequences of this land-sea divide for coastal terrain modelling. We study how methods designed for terrestrial environments can be used to create integrated raster coastal terrain models (CTMs) from coarse elevation and depth data. We focus on shore slopes, because many particularities of coastal terrain and the data which describe it as well as the resulting problems are concentrated in the shore zone. Based on shorelines, terrestrial contours, depth contours and depth points, we used the ANUDEM algorithm to interpolate CTMs at different spatial resolutions, with and without drainage enforcement, for two test areas in a highly complex archipelago coast. Slope aspect and gradient rasters were derived from the CTMs using Horn's algorithm. Values were assigned from the slope rasters to thousands of points along the test areas' shorelines in different ways. Shore slope gradients and aspects were also calculated directly from the shorelines and contours. These modelled data were compared to each other and to field-measured shore profiles using a combination of qualitative and quantitative methods. As far as the coarse source data permitted, the interpolation and slope calculations delivered good results at fine spatial resolutions. Vector-based slope calculations were very sensitive to quality problems of the source data. Fine-resolution raster data were consequently found most suitable for describing shore slopes from coarse coastal terrain data. Terrestrial and marine parts of the CTMs were subject to different errors, and modelling methods and parameters had different consequences there. Thus, methods designed for terrestrial applications can be successfully used for coastal terrain modelling, but the choice of methods and parameters and the interpretation of modelling results require special attention to the differences of terrestrial and marine topography and data.     

数字地形分析在滑坡研究中的应用综述

高效的数字地形分析（Digital Terrain Analysis,DTA）是滑坡预测与评估研究的重要手段。文章综述了DTA在滑坡研究中的应用现状,基本内容包括地形因子分析、地形形态分析、地形单元划分以及DEM与滑坡模型的结合分析。地形因子分析的应用多而广,主要思路是在地形因子与滑坡发育的关系研究基础上分析其滑坡敏感性,进而构建滑坡预测和评估模型;地形形态分析是滑坡识别的重要手段,加强地貌形态和滑坡发育的关系研究有助于对潜在滑坡地形的识别;地形单元划分能为滑坡研究提供统计和分析单元;DEM与滑坡专业模型的结合方式多样,程度各异。同时,从尺度选择与转换的角度探讨了DTA滑坡研究的尺度问题,分析了DTA的局限性,指出DEM不能提供完备无误的地形信息,DTA不能完全取代常规的地形分析。最后,基于以上论述对未来的研究趋势提出了展望。     

Evaluation on the accuracy of digital elevation models

1 IntroductionDigital elevation model (DEM) is digital representation of relief. It is one of the most important components in the database of GIS. At present, DEM is playing a key role in the field of survey and mapping, remote sensing and almost all the terrain related geographical analyses. DEM can be grouped into regular grids (raster) and triangulated irregular networks (TIN). Both have their advantages and disadvantages in application. It is generally believed that grid DEM will …     

Evaluation on the accuracy of digital elevation models

There is a growing interest in investigating the accuracy of digital elevation model (DEM). However people usually have an unbalanced view on DEM errors. They emphasize DEM sampling errors, but ignore the impact of DEM resolution and terrain roughness on the accuracy of terrain representation. This research puts forward the concept of DEM terrain representation error (Et) and then investigates the generation, factors, measurement and simulation of DEM terrain representation errors. A multi-resolution and multi-relief comparative approach is used as the major methodology in this research. The experiment reveals a quantitative relationship between the error and the variation of resolution and terrain roughness at a global level. Root mean square error (RMS Et) is regressed against surface profile curvature (V) and DEM resolution (R) at 10 resolution levels. It is found that the RMS Et may be expressed as RMS Et = (0.0061 × V+ 0.0052) × R - 0.022 × V + 0.2415. This result may be very useful in forecasting DEM accuracy, as well as in determining the DEM resolution related to the accuracy requirement of particular application.