三角网的生成与简化算法

在三维地理信息系统的开发与研究中 ,要求三角网模型的支持 ,而高效的三角网生成算法是三角网模型的基础。该文综述各类三角网的生成算法 ,对以往被人忽视但简明高效的算法———径向扫描算法进行阐释 ,并对它的实现进行改进 ;对地形数据的简化及建立地形层次对象模型是实现高效的三维地理信息系统的关键 ,而三角网的简化是地形数据简化的基石之一 ,文中提供三种简化三角网的算法供同行讨论 ;并在微机上实现上述算法。     

Quantifying positional error induced by line simplification

This study examines the effects of line simplification on the positional accuracy of linear features. The goal is to quantify the relation between the level of simplification and the degree of positional error, so that users can choose appropriate levels of simplification that will yield results meeting specific accuracy criteria. The study focuses on the Douglas-Peucker line simplification algorithm and examines both natural and anthropogenic features (streams and roads) derived from United States Geological Survey Digital Line Graphs. Results show that error can be modelled at an aggregate level using cumulative frequency curves and their confidence limits. This makes it possible to identify the level of simplification that eliminates the largest number of vertices while still attaining a specific positional accuracy standard. A simple implementation strategy is described in which an optimal level of simplification is identified and simplification is applied selectively for different lines. The study shows that management of simplification induced error is possible using simple tools well within the reach of GIS users.     

一种基于面积平衡约束的图斑化简算法

保持综合前后各地类土地面积平衡是土地利用数据综合的基本原则.地类图斑是面积统计的最小单元,用常规的线化简算法不但难以保证图斑的地理特性,而且会给面积统计带来不确定性.该文提出一种能够使图斑化简前后面积保持绝对平衡的化简算法,该算法给出了求取图斑群和岛图斑的地理特征点、几何特征点的相关方法,对被特征点分割的综合子弧段运用直线面积平衡和参数曲线面积平衡两种求取平衡线的方式进行化简,基本原理是综合前后弧段切割它们自身的包络矩形面积比相等,且综合后的弧段须落入综合前子弧段的误差缓冲区内.实验表明,该算法不但能保持图斑的形态特征,而且能保持图斑化简前后面积绝对相等.     

Knowledge-guided consistent correlation analysis of multimode landslide monitoring data

A novel method called knowledge-guided spatio-temporal consistent correlation analysis (KSTCCA) was developed to discover reliable deformation features induced by multiple factors based on multimode landslide monitoring data. Compared to conventional approaches, KSTCCA integrates both temporal and spatial correlation analysis to improve the consistency of deformation patterns and capture the spatio-temporal heterogeneities in multimode monitoring data. KSTCCA considers both the landslide deformation mechanisms and the relationships between different influential factors as knowledge. Moreover, the method extracts the morphological structures of monitoring curves based on a seven-point approach and identifies knowledge rules using the k-means clustering method. Under the guidance of prior knowledge, a spatial correlation analysis is conducted based on support vector regression, and a temporal correlation analysis of the time lag is carried out based on the morphological structure features. Finally, three kinds of typical monitoring data, including deformation, rainfall, and reservoir water level data collected in the Baishuihe landslide area, China, are used for experimental analysis to verify the validity of the proposed method.     

A framework of spatio-temporal trajectory simplification methods

We present an extensible, generic, spatio-temporal trajectory simplification framework that modularises trajectory simplification into the stages of normalising, ranking, and reduction. We combine a range of ranking strategies and scoring heuristics – some from the literature and some new – into our framework modules and create a variety of spatio-temporal trajectory simplification methods. These trajectory simplification methods are experimented upon using real world and synthetic datasets, measuring running time, geometric displacement, and region-of-interest visitation. The results indicate that our proposed framework creates a number of efficient and effective spatio-temporal trajectory simplification methods.     

Rendering interior-filled polygonal vector data in a virtual globe

Polygonal vector data are important for representing countries, lakes, residential settlements, and other polygonal features. The proper representation of polygonal vector data is the basis of efficient rendering and picking and quick access and display of the analysis results based on polygons (e.g., 3D overlaying and surface area measurement in mountainous areas) in a virtual globe. However, polygonal vector data are displayed using texture-based or boundary-based approaches in most existing virtual globes. The texture-based approach cannot easily support interactive operations (e.g., picking) and spatial analysis (e.g., adjacency analysis and spatial measurement). The boundary-based approach treats the holes as independent features; however, it is difficult to recognize which boundaries constitute a polygon. Further research is needed on how to better organize the polygons to support efficient rendering, picking, and analysis in a virtual globe. In this article, we propose two methods to drape interior filled 2D polygons onto a multi-resolution 3D terrain. Both proposed methods combine polygon clipping and polygon triangulation. The difference between the two methods is in the way holes are eliminated. Method 1 recursively subdivides a terrain triangle until the child-triangles contain no holes; every resulting clipped polygon, which is then triangulated, contains no holes. Method 2 directly clips a polygon against a terrain triangle and creates bridge edges to transform the resulting polygons with holes to degenerate polygons that are further triangulated. The experimental results demonstrate that both proposed methods can efficiently process polygons with holes resulting in appropriate numbers of triangles. The processed interior-filled polygons remain close to the terrain surface in a virtual globe. Both proposed methods support real-time rendering of polygonal vector data in a virtual globe.     

Envelope generation and simplification of polylines using Delaunay triangulation

As a basic and significant operator in map generalization, polyline simplification needs to work across scales. Perkal’s ε-circle rolling approach, in which a circle with diameter ε is rolled on both sides of the polyline so that the small bend features can be detected and removed, is considered as one of the few scale-driven solutions. However, the envelope computation, which is a key part of this method, has been difficult to implement. Here, we present a computational method that implements Perkal’s proposal. To simulate the effects of a rolling circle, Delaunay triangulation is used to detect bend features and further to construct the envelope structure around a polyline. Then, different connection methods within the enveloping area are provided to output the abstracted result, and a strategy to determine the best connection method is explored. Experiments with real land-use polygon data are implemented, and comparison with other algorithms is discussed. In addition to the scale-specificity inherited from Perkal’s proposal, the results show that the proposed algorithm can preserve the main shape of the polyline and meet the area-maintaining constraint during large-scale change. This algorithm is also free from self-intersection.     

曲线化简的弧比弦算法改进及其评价

地图综合是当前国际GIS和制图学领域的一个难题。化简是地图综合的一个基本操作算子,线要素是地图综合操作的主要对象,该文主要研究地图线要素的化简问题。首先,从算法的适用范围、阈值设置等方面分析了现有算法的优点,重点阐述了一种常用的弧比弦算法,并指出了其在化简过程中存在的不足。然后,基于地图曲线的空间认知特征,提出了一种改进弧比弦算法。该算法能够更好地保持曲线的特征点和曲线的几何形态,避免阈值的设置问题。在算法评价上,采用3种定量评价指标,即:偏移平均值、偏移量中值和面积变形值,对改进的弧比弦算法与现有经典算法进行了比较分析。最后,通过算例分析验证了该算法的可行性、有效性和健壮性。     

A new methodology for measuring coastline recession using buffering and non‐linear least squares estimation

Coastline recession is one of the best indicators of coastal erosion. Three methods for computing coastline recession – the baseline approach, the dynamic segmentation approach and the area‐based approach – have been used, each of which has one or more drawbacks. To overcome these problems, a new methodology for measuring coastline recession is proposed, using buffering and non‐linear least squares estimation. The proposed method was compared with the three existing methods with respect to two simulated cases and two real coastlines. Test results confirmed that the new method is more reliable than the three other methods, all of which are susceptible to variability of recession, scale, number of line segments, length of coastlines and direction of the baseline. The proposed method, incorporating two physically meaningful values – magnitude and variability of coastline recession according to the mean and standard deviation of coastline offsets, respectively – presents itself as an effective alternative method of assessing coastline recession.     

城市三维地理信息系统中三维模型的快速构建方法

大规模三维模型的快速构建一直是影响城市三维地理信息系统(3DUGIS)发展的一个重要因素.基于3DUGIS中景观模型的表达原则与分类,分别针对抽象的点、线、面状对象提出符号匹配和三角剖分的批量三维模型构建方法.这种建模方法方便、快捷、自动化程度高,可广泛应用于城市景观中地面、河流、道路、绿地、规则建筑物、地下管线等地物的三维建模.通过对上述三维模型快速构建方法的实现,验证了三维模型生成的效果和效率.     

大规模地形漫游中的实时LOD算法研究

大规模地形漫游在游戏、仿真、虚拟现实等领域有着广泛的应用。该文在总结现有地形简化算法的基础上,提出了一种基于动态调度的地形块内视点相关二叉树简化算法,有效地简化地形,实现大规模场景的实时漫游。     

A scale-explicit model for checking directional consistency in multi-resolution spatial data

Multi-resolution spatial data always contain the inconsistencies of topological, directional, and metric relations due to measurement methods, data acquisition approaches, and map generalization algorithms. Therefore, checking these inconsistencies is critical for maintaining the integrity of multi-resolution or multi-source spatial data. To date, research has focused on the topological consistency, while the directional consistency at different resolutions has been largely overlooked. In this study we developed computation methods to derive the direction relations between coarse spatial objects from the relations between detailed objects. Then, the consistency of direction relations at different resolutions can be evaluated by checking whether the derived relations are compatible with the relations computed from the coarse objects in multi-resolution spatial data. The methods in this study modeled explicitly the scale effects of direction relations induced by the map generalization operator – merging, thus they are efficient for evaluating consistency. The directional consistency is an essential complement to topological and object-based consistencies.     

Comparative assessment of three approaches for deriving stream power plots along long profiles in the upper Hunter River catchment, New South Wales, Australia

The downstream distribution of stream power is derived and analysed for 11 different streams in the upper Hunter River catchment, Australia. Stream long profiles were produced in a GIS environment using DEM data and catchment area–discharge analysis. These profiles were analysed using three approaches, namely long profile smoothing, curve fitting and a theoretical model. The methodology for deriving stream power profiles using these three approaches is discussed. The long profile smoothing method provides a good approximation of the subcatchment variability in stream power trends. The curve fitting method shows that higher-order exponential curves provide a better fit for long profile data. For the streams of the upper Hunter River catchment, second-order exponential curves fit well with significantly less error. The curve fitting method predicts a bimodal (upstream and midstream) distribution of stream power, which is a deviation from our earlier understanding of a single midstream peak. The theoretical approach provides a mathematical expression of the observed bimodal stream power distribution. The bimodal distribution emphasises the erosion potential of headwater reaches. The resultant stream power distribution provides a catchment-scale characterisation of the distribution of available energy in any given system. Using these approaches, the variability of stream power in headwater reaches is explained by discharge variability, while variability in midstream and downstream reaches is related to high variability in channel gradient.     

Multi‐visibility maps of triangulated terrains

Visibility computation on terrain models is an important research topic with many applications in Geographical Information Systems. A multi‐visibility map is the subdivision of the domain of a terrain into regions that, according to different criteria, encodes the visibility with respect to a set of view elements. We present an approach for visualising approximated multi‐visibility maps of a triangulated terrain corresponding to a set of view elements by using graphics hardware. Our method supports heterogeneous sets of view elements containing points, segments, polygonal chains and polygons and works for weak and strong visibility. Moreover, we are also able to efficiently solve approximated point and polygonal region multi‐visibility queries. To illustrate the usefulness of our approach we present results obtained with an implementation of the proposed algorithms.     

新疆渭干河流域地下水含盐量的时空变异特征

对面积约3 408 km²的渭干河流域地下水含盐量进行取样检测,应用地质统计学方法对取得数据进行了半方差函数分析。结果表明,该地区1998、1999年地下水含盐量变异函数曲线的理论模型符合球状模型;1997年、2000年地下水含盐量变异函数曲线的理论模型符合有基台线性模型。通过对1997年、1998年、1999年及2000年水样的分析,得出该地区地下水含盐量在时间与空间上皆存在明显变异性的结论。在空间尺度上,地下水含盐量从研究区的西北向东南方向有增加的趋势。从整体上看,研究区地下水含盐量东部高于西部;南部高于北部;东南部高于西北部。在时间尺度上,研究区东南部地下水随着时间的推移正向盐渍化加重方向发展。本文利用软件Geopack与Surfer7.0软件绘制了地下水含盐量的时空分布图,为该地区地下水盐分的管理与今后的防止盐渍化提供了决策依据。     

基于NSCT与改进SIFT特征点的GF-2影像水印算法

现有基于SIFT特征点的水印算法因特征区域重叠导致算法鲁棒性较差,不能满足GF-2影像版权保护的需求。该文采用Mean Shift对SIFT特征点进行优化和改进,提出一种基于NSCT与改进SIFT特征点的GF-2影像数字水印算法。首先,提取GF-2影像的SIFT特征点,采用Mean Shift对其进行聚类处理,将所有聚类中心作为影像的关键点,并计算关键点的平均SIFT描述符,以保证所生成的影像关键点具有与SIFT特征点相同的特征属性;其次,根据关键点构建影像的特征区域,并对其进行几何归一化处理;最后,对特征区域进行NSCT分解,选择低频子带进行奇异值分解,根据加性规则将水印信息的奇异值嵌入低频子带的奇异值中,并通过相应的逆变换得到含水印影像。与其他算法对比验证结果表明,该算法既具有良好的不可见性,又对常规攻击以及旋转、裁剪、缩放等几何攻击具有较好的鲁棒性,且能有效减轻特征区域的重叠现象,适用于GF-2影像的版权保护。     

Terrestrial surface-area increment: the effects of topography,DEM resolution,and algorithm

The area increment of land surface compared with its projected area is an effect of topographic relief and is also a source of environmental variations. To examine the effects of topography and data resolution on surface area calculation, we calculated incremental area coefficients (IACs), based on two different algorithms, for a DEM of China at a series of spatial resolutions. Sampling the DEM with a regional network of 50?km?×?50?km cell size, we explored the relationships among the two IACs and topographic features. Both IACs studied were exponential functions of resolution. At 30-m resolution, the IACs were 4.31 and 4.89% over China, respectively. The largest increment for a 50?km?×?50?km cell was >45%. Between the IACs there was a linear relationship that varied with DEM resolution. Hierarchical variation partitioning revealed that the factors included contributed in a very similar percentage composition to the two IACs, mean slope (37.5 or 38.7%) and standard deviation of slope (22.3 or 19.6%) at local scale dominated the area increment, followed by regional elevation range. Data resolution contributed about 10%, while the deviation of slope exposure only had minimal (1.4 or 1.7%) impact on surface-area increment. For a specific type of geomorphology, a threshold resolution of DEM can be determined, below which the surface-area increment (i.e., IAC) is negligible. Our results provided the first comprehensive estimate of the contributions of the topographic features, DEM resolution, and algorithms for the surface-area increment, and indicated the scale-related properties and potential environmental consequences of topographic heterogeneity in various estimates of natural resources and ecosystem functions when area needs to be taken into account.     

A Multiresolution topographic surface database

Abstract

Multiresolution data structures provide a means of retrieving geographical features from a database at levels of detail which are adaptable to different scales of representation. A database design is presented which integrates multi-scale storage of point, linear and polygonal features, based on the line generalization tree, with a multi-scale surface model based on the Delaunay pyramid. The constituent vertices of topologically-structured geographical features are thus distributed between the triangulated levels of a Delaunay pyramid in which triangle edges are constrained to follow those features at differing degrees of generalization. Efficient locational access is achieved by imposing a spatial index on each level of the pyramid.     

Geometric-based approach for integrating VGI POIs and road networks

Integrating heterogeneous spatial data is a crucial problem for geographical information systems (GIS) applications. Previous studies mainly focus on the matching of heterogeneous road networks or heterogeneous polygonal data sets. Few literatures attempt to approach the problem of integrating the point of interest (POI) from volunteered geographic information (VGI) and professional road networks from official mapping agencies. Hence, the article proposes an approach for integrating VGI POIs and professional road networks. The proposed method first generates a POI connectivity graph by mining the linear cluster patterns from POIs. Secondly, the matching nodes between the POI connectivity graph and the associated road network are fulfilled by probabilistic relaxation and refined by a vector median filtering (VMF). Finally, POIs are aligned to the road network by an affine transformation according to the matching nodes. Experiments demonstrate that the proposed method integrates both the POIs from VGI and the POIs from official mapping agencies with the associated road networks effectively and validly, providing a promising solution for enriching professional road networks by integrating VGI POIs.