Elevation models for reproducible evaluation of terrain representation

ABSTRACT

This paper proposes elevation models to promote, evaluate, and compare various terrain representation techniques. Our goal is to increase the reproducibility of terrain rendering algorithms and techniques across different scales and landscapes. We introduce elevation models of varying terrain types, available to the user at no cost, with minimal common data imperfections such as missing data values, resampling artifacts, and seams. Three multiscale elevation models are available, each consisting of a set of elevation grids, centered on the same geographic location, with increasing cell sizes and spatial extents. We also propose a collection of single-scale elevation models of archetypal landforms including folded ridges, a braided riverbed, active and stabilized sand dunes, and a volcanic caldera. An inventory of 78 publications with a total of 155 renderings illustrating terrain visualization techniques guided the selection of landform types in the elevation models. The benefits of using the proposed elevation models include straightforward comparison of terrain representation methods across different publications and better documentation of the source data, which increases the reproducibility of terrain representations.     

Terrain corrections to power 3 in gravimetric geoid determination

In precise geoid determination by Stokes formula, direct and primary and secondary indirect terrain effects are applied for removing and restoring the terrain masses. We use Helmert's second condensation method to derive the sum of these effects, together called the total terrain effect for geoid. We develop the total terrain effect to third power of elevation H in the original Stokes formula, Earth gravity model and modified Stokes formula. It is shown that the original Stokes formula, Earth gravity model and modified Stokes formula all theoretically experience different total terrain effects. Numerical results indicate that the total terrain effect is very significant for moderate topographies and mountainous regions. Absolute global mean values of 5–10 cm can be reached for harmonic expansions of the terrain to degree and order 360. In another experiment, we conclude that the most important part of the total terrain effect is the contribution from the second power of H, while the contribution from the third power term is within 9 cm. Received: 2 September 1996 / Accepted: 4 August 1997     

A comparison of three image-object methods for the multiscale analysis of landscape structure

Within the conceptual framework of Complex Systems, we discuss the importance and challenges in extracting and linking multiscale objects from high-resolution remote sensing imagery to improve the monitoring, modeling and management of complex landscapes. In particular, we emphasize that remote sensing data are a particular case of the modifiable areal unit problem (MAUP) and describe how image-objects provide a way to reduce this problem. We then hypothesize that multiscale analysis should be guided by the intrinsic scale of the dominant landscape objects composing a scene and describe three different multiscale image-processing techniques with the potential to achieve this. Each of these techniques, i.e., Fractal Net Evolution Approach (FNEA), Linear Scale-Space and Blob-Feature Detection (SS), and Multiscale Object-Specific Analysis (MOSA), facilitates the multiscale pattern analysis, exploration and hierarchical linking of image-objects based on methods that derive spatially explicit multiscale contextual information from a single resolution of remote sensing imagery. We then outline the weaknesses and strengths of each technique and provide strategies for their improvement.     

Digital Elevation Model from the Best Results of Different Filtering of a LiDAR Point Cloud

The LiDAR point clouds captured with airborne laser scanning provide considerably more information about the terrain surface than most data sources in the past. This rich information is not simply accessed and convertible to a high quality digital elevation model (DEM) surface. The aim of the study is to generate a homogeneous and high quality DEM with the relevant resolution, as a 2.5D surface. The study is focused on extraction of terrain (bare earth) points from a point cloud, using a number of different filtering techniques accessible by selected freeware. The proposed methodology consists of: (1) assessing advantages/disadvantages of different filters across the study area, (2) regionalization of the area according to the most suitable filtering results, (3) data fusion considering differently filtered point clouds and regions, and (4) interpolation with a standard algorithm. The resulting DEM is interpolated from a point cloud fused from partial point clouds which were filtered with multiscale curvature classification (MCC), hierarchical robust interpolation (HRI), and the LAStools filtering. An important advantage of the proposed methodology is that the selected landscape and datasets properties have been more holistically studied, with applied expert knowledge and automated techniques. The resulting highly applicable DEM fulfils geometrical (numerical), geomorphological (shape), and semantic quality properties.     

Visibility Analysis with the Multiscale Implicit TIN

Visibility analysis is now a key function of many geographical information systems. It is also one of the most contentious tools, as it is notoriously prone to error. The paper will demonstrate the versatility of the Multiscale Implicit Triangulated Irregular Network (TIN) for the application of intervisibility analysis at multiple resolutions. This approach allows for the integration of three-dimensional (3D) topographic features with the terrain surface. The multiscale TINs are derived from generalising digital contours at a variety of lateral tolerances. The models' performances are evaluated from an extensive field study undertaken in the South Wales valleys. Results suggest that the accuracy of intervisibility analysis is very dependent upon the availability of good quality 3D topographic data. In our study, such data were shown to improve visibility performance by more than 44% over its bare-earth TIN equivalent. Interestingly, generalisation of the TINs had very little effect on visibility performance. In addition, a Monte Carlo approach to sensitivity analysis was found to be detrimental to the accuracy of visibility prediction in the full terrain and topographic models. However, this probable approach can improve intervisibility performance by up to 18% on a bare-earth TIN. The range of these visibility modelling scenarios demonstrate the flexibility of the Multiscale Implicit TIN for digital surface modelling.     

Cartographically Wordy but not Necessarily Worthy

Abstract

A novel method called multidirectional visibility index (MVI) has been developed and verified. The MVI improves standard cartographic analytical shading with a number of enhancements to topographic detail and prominent structures, i.e. the portrayal of flat areas in lighter tones, the accentuation of morphologic edges, and the multiscale visualisation of morphologic terrain features. The procedure requires a digital elevation model (DEM) and involves the following steps: visibility mask computation; the respective multidirectional altering of the azimuth and elevation angle; the generation of continuous grid MVIs that indicate upper/lower views, quasi-slope, and relative relief; and an appropriate visualisation of the relevant MVI as a standalone technique or in combination with standard hill-shaded relief. The modelling parameters are robust and therefore highly adaptive to different landforms.     

三维地形可视化技术研究进展

三维地形是虚拟自然环境中不可缺少的因素,也是虚拟仿真领域中视景系统的重要组成部分。该文将地形生成技术概括为三维地形简化技术、大规模地形数据的动态调度与地形简化相结合的处理技术和地形纹理数据的组织与应用技术等三大类,并分别介绍了这三类技术的相关理论和方法。     

增量法地形可视计算与分析

DEM在表达地形时具有水平等距网格和垂直高差的特点,根据DEM表达地形的特点,利用双增量地形可视计算方法,以水平方向矢量叉乘增量计算来判断点的可视性,采用垂直方向高程增量的谷地凹处盲区测试,根据地形高程的变化来消除不可视点,该算法思路清晰、计算复杂度低。同时重点分析该算法在地形的部分区域和不同特征地形上的特点,指出该算法在起伏山地和沟谷地形中的优势;横向上与ArcGIS功能模块进行比较,可视计算速度大大提高。     

A Uniform Sky Illumination Model to Enhance Shading of Terrain and Urban Areas

Users of geographic information systems (GIS) usually render terrain using a point light source defined by an illumination vector. A terrain shaded from a single point provides good perceptual cues to surface orientation. This type of hill shading, however, does not include any visual cues to the relative height of surface elements. We propose shading the terrain under uniform diffuse illumination, where light arrives equally from all directions of a theoretical sky surrounding the terrain. Surface elements at lower elevations tend to have more of the sky obscured from view and are thus shaded darker. This tinting approach has the advantage that it provides more detailed renderings than point source illumination. We describe two techniques of computing terrain shading under uniform diffuse illumination. One technique uses a GIS–based hill-shading and shadowing tool to combine many point source renderings into one approximating the terrain under uniform diffuse illumination. The second technique uses a C++ computer algorithm for computing the inclination to the horizon in all azimuth directions at all points of the terrain. These virtual horizons are used to map sky brightness to the rendering of the terrain. To evaluate our techniques, we use two Digital Elevation Models (DEMs)—of the Schell Creek Range of eastern Nevada and a portion of downtown Houston, Texas, developed from Light Detection and Ranging (lidar) data. Renderings based on the uniform diffuse illumination model show more detailed changes in shading than renderings based on a point source illumination model.     

DEM地形信息提取对比研究——以坡度为例

由于DEM数据本身多尺度因素,加之地形、地貌特征具有宏观性与区域分异性的特点,直接的信息提取往往很难达到预期的目的。利用DEM制作坡度图高效、省力,但其精度有很大的不确定性,同时DEM制作过程中的误差传播、转移对坡度信息的影响缺少系统的判断依据。选取位于陕北黄土高原上的两个不同地区作为实验样区,在不同DEM生产的基础上,以高精度的1∶10 000DEM为准值,通过对1∶5万和1∶1万DEM提取定量地形要素的叠合、比较与统计分析,探讨具有不同地貌类型的区域1∶5万DEM提取地形信息的精度及其统计意义上的数量百分比关系。     

城市地形图动态修补测工作量调查方法初探

在城市建设日新月异开展的形势下,全国各城市基层国土资源部门开展城市地形动态修补测工作。本文结合实例提出了一种城市大比例尺地形图动态修补测工作量普查的方法,对涉及到地形地貌变化的城市动态修补测的工作量能够较准确进行预测,大大提高了政府动态修补测资金的利用效率,对城市动态修补测工作量调查有一定的参照意义。     

A Multiscale Object-Specific Approach to Digital Change Detection

Landscape spatial pattern is dependent not only on interacting physiographic and physiological processes, but also on the temporal and spatial scales at which the resulting patterns are assessed. To detect significant spatial changes occurring through space and time three fundamental components are required. First, a multiscale dataset must be generated. Second, a change detection framework must be applied to the multiscale dataset. Third, a procedure must be developed to delineate individual image-objects and identify them as they change through scale. In this paper, we introduce an object-specific multiscale digital change detection approach. This approach incorporates multitemporal SPOT Panchromatic (Pan) data, object-specific analysis (OSA), object-specific up-scaling (OSU), marker-controlled watershed segmentation (MCS) and image differencing change detection. By applying this framework to SPOT Pan data, image-objects that have changed between registration dates can be identified and delineated at their characteristic scale of expression. Results illustrate that this approach has the ability to automatically detect changes at multiple scales as well as suppress sensor related noise. This study was conducted in the forest region of the Örebro Administrative Province, Sweden.     

Exploring uncertainties in terrain feature extraction across multi-scale,multi-feature,and multi-method approaches for variable terrain

Terrain analysis uses different workflows to extract features from terrain models for the purpose of understanding topographic patterns and processes. However, the results of different workflows often conflict, leading to uncertainties about feature locations. Instead of relying upon a single workflow, we suggest that a fusion of information from multiple workflows better informs terrain analysis. From terrain data with different degrees of variability, we extracted terrain features related to the set of topographic surface network feature classes {peaks, pits, saddles, ridges, courses} using workflows from free, open-source, and commercial software. A multi-scale analysis produced terrain features with fuzzy membership values for various feature classes and revealed that terrain locations can exhibit characteristics of all classes. Multi-feature maps were created by determining at each location the dominant and second-ranked features, and an uncertainty value. Our multi-method approach incorporated all of the workflows’ multi-scale results and again produced multi-feature maps that increased the confidence of some features and reduced the signal of dissimilar results. We also found that high variability terrain produced crisper features in both spatial extent and membership strength. Our overall conclusion is that multi-scale, multi-feature, and multi-method analyses clarify terrain feature uncertainty.     

重力数据的多尺度边缘分析及其应用(英文)

In order to improve the processing and interpretation of gravity data, multiscale edge theory in image processing is introduced into the study of gravity field. Multiscale edges of gravity anomaly are analyzed based on a special wavelet. It shows that the multiscale edges are the extrema points of the horizontal gravity gradient at different heights, which are related to the sharp discontinuities of underground sources. The applications of multiscale edge in downward continuation and gravity inversion are discussed. The simulated examples show that the multiscale edges can be applied to stabilize the downward continuation operator when the continuation height is low. The multiscale edges also have a convenient application to infer the geometry of the underground source. Moreover, the gravity inversion algorithm based on the multiscale edges has a good antinoise property. Supported by the National Natural Science Foundation of China(No.40704003), the National 973 Program of China(No.2007CB714405), the Open Research Fund from Key Laboratory of Geospace Environment and Geodesy(No.04-01-08).     

基于共轭地表曲面的山脊线和山谷线提取方法的研究

从数字化地形资料中自动提取山脊线和山谷线的技术,在测绘、工程设计等方面有着重要的意义。本文对地形流水模拟法提取山脊线和山谷线的方法进行深入研究后,提出利用共轭地表曲面的概念实现自动提取算法山谷线和山脊线的方法,并设计出了基于共轭地表曲面的地形流水模拟法自动提取算法山谷线和山脊线的算法。文后通过实验验证了该方法的有效性,它所提取的山脊线和山谷线与实际地形相符合。     

3维地形的金字塔上下采样局部实时简化算法

在大型三维地理信息系统中,往往需要在预加载整体地形数据的基础上,对局部地形数据重新构建三角网,用于物理模拟等应用。针对这一问题,本文提出了一种三维地形的金字塔“上下采样”局部实时简化算法,实现对局部三角网地形实时高效的简化。该算法基于图像金字塔思想,通过对原始地形的三角格网执行“下采样”和“上采样”,结合“预测残差”对“下采样”的地形做局部简化,简化后的三角网十分逼近原始的高精度三角网。本文将该算法用于GeoBeans 3D平台的汽车驾驶模拟系统中。该算法生成的地形简化且稳定,随着汽车活动范围的变化,按需销毁及重构更新地形。实验证明,当预测残差阈值取0.1时,三角形个数能简化到原始数量的2/3;阈值取0.2时,三角形个数可以简化到原始个数的1/4左右。由此可见,在视觉精度允许的范围之内,该算法对地形的简化效果较好,且耗时少,满足了三维系统及车辆驾驶模拟的实时性。     

Polarimetric SAR Data Correction and Terrain Topography Measurement Based on the Radar Target Orientation Angle

Topographic variations caused by the range and the azimuth terrain slopes induce polarization orientation changes which cause the polarization to rotate about the line of sight. The existence of these variations reduce the accuracy measurement of geophysical parameters from polarimetric synthetic aperture radar (PolSAR) images. For this reason most inversion studies are best done in area of flat earth. In area which has significant terrain variations require compensation for topography. In real situations, terrain slopes rotate the polarization basis of the polarimetric scattering matrices by an orientation angle shift, and induce significant cross-polarization power. In this paper, two methods have been investigated using the polarimetric orientation angle (PAO): the first one involves the rotation of the polarimetric scattering and coherency matrices to achieve maximum azimuthally asymmetry for polarimetric data compensation to ensure accurate estimation of geophysical parameters in rugged terrain areas. The second approach has been developed which measures azimuth and range terrain slopes that are related to shifts in polarization orientation angle. Terrain elevation maps relative to a plane parallel to the radar line of sight can then be generated by integrating these slopes requiring only one PolSAR flight pass by combing orientation angle estimation and a shape-from-shading technique (SFS) which is mostly used by the computer vision community. Experimental results with C-band polarimetric RADARSAT2 data are used evaluate the data compensation algorithm and DEM generation.     

Allied Military Model Making during World War II

It is generally accepted that the three-dimensional nature of the digital terrain model enhances our visualization of surfaces. Modern techniques enable a detailed landscape to be constructed as a facsimile of reality that provides an opportunity to move through or fly over the landscape. Given these benefits, it is little surprise that simulations using digital terrain models are employed as essential visual aids for briefing and training military personnel prior to land, air, and sea operations. Though these capabilities are significant, they are not necessarily, in the basic sense, new. This paper traces the development and examines the role of terrain models made by the Allies during World War II, a period prior to the development of computer-based modeling. Though made from basic materials, these sophisticated terrain models were hand crafted by enlisted sculptors, architects, stage designers, and artists, who carefully modeled a sculpture of the landscape to be an invaluable aid during key military operations of World War II.     

地貌晕渲生成技术分析

通过对地貌晕渲生成技术方法的研究,重点讲述了ArcInfo和Atlas3D软件在生成地貌晕渲的主要过程,并分析了各自的特点,旨在通过技术交流,提高地貌晕渲图的生产效益和产品质量,     

The Effect of DEM Raster Resolution on First Order, Second Order and Compound Terrain Derivatives

It is well known that the grid cell size of a raster digital elevation model has significant effects on derived terrain variables such as slope, aspect, plan and profile curvature or the wetness index. In this paper the quality of DEMs derived from the interpolation of photogrammetrically derived elevation points in Alberta, Canada, is tested. DEMs with grid cell sizes ranging from 100 to 5 m were interpolated from 100 m regularly spaced elevation points and numerous surface‐specific point elevations using the ANUDEM interpolation method. In order to identify the grid resolution that matches the information content of the source data, three approaches were applied: density analysis of point elevations, an analysis of cumulative frequency distributions using the Kolmogorov‐Smirnov test and the root mean square slope measure. Results reveal that the optimum grid cell size is between 5 and 20 m, depending on terrain com‐plexity and terrain derivative. Terrain variables based on 100 m regularly sampled elevation points are compared to an independent high‐resolution DEM used as a benchmark. Subsequent correlation analysis reveals that only elevation and local slope have a strong positive relationship while all other terrain derivatives are not represented realistically when derived from a coarse DEM. Calculations of root mean square errors and relative root mean square errors further quantify the quality of terrain derivatives.