SOFTWARE REVIEWS

Electromap World Atlas , Version 1.1. Interactive Population Statistical System , Version 1.0. Jerry W. Wicks and Jose Luiz Pereira de Almeida MATHCAD , Version 2.5. Memory Mate. Micro DEM +, Version 5.21. Peter Guth NCSS – Number Cruncher Statistical System , Version 5.02. Dr. Jerry L. Hintze     

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。     

基于DTM的黄土丘陵沟壑区太阳辐射值计算模型及应用研究

以地形特征及周围地形遮蔽状况、经纬度位置等作为太阳辐射空间分布差异的变量，在地理信息系统（GIS）支持下，建立黄土丘陵沟壑区数字高程模型（DTM）和太阳辐射值计算模型，并实现理论太阳辐射值空间分布的可视化表达。     

机载激光雷达(LiDAR)测量在公路三维测设中的应用

精密数字地面模型(DTM)是进行公路三维测设的基础。吉林省东部山区地形复杂,植被茂盛,传统全野外测量获取DTM难度大、周期长、精度低。从2011年起,我院开始利用机载Li DAR技术获取DTM数据产品,成功应用于7条高等级公路的勘测设计中,证明了在四季分明的广大北方地区机载Li DAR在公路三维测设中具有较好的实用性和应用前景。     

利用DTM法进行土石方工程质量分析

DTM法越来越多地被应用于土石方工程的计算中,但对于它的研究仍局限于挖填方计算,对土石方工程质量分析却鲜有论述,本文从需求和应用两个方面对利用DTM法的土石方工程质量分析进行介绍。     

Evaluation of on-line DEMs for flood inundation modeling

Recent and highly accurate topographic data should be used for flood inundation modeling, but this is not always feasible given time and budget constraints so the utility of several on-line digital elevation models (DEMs) is examined with a set of steady and unsteady test problems. DEMs are used to parameterize a 2D hydrodynamic flood simulation algorithm and predictions are compared with published flood maps and observed flood conditions. DEMs based on airborne light detection and ranging (LiDAR) are preferred because of horizontal resolution, vertical accuracy (∼0.1 m) and the ability to separate bare-earth from built structures and vegetation. DEMs based on airborne interferometric synthetic aperture radar (IfSAR) have good horizontal resolution but gridded elevations reflect built structures and vegetation and therefore further processing may be required to permit flood modeling. IfSAR and shuttle radar topography mission (SRTM) DEMs suffer from radar speckle, or noise, so flood plains may appear with non-physical relief and predicted flood zones may include non-physical pools. DEMs based on national elevation data (NED) are remarkably smooth in comparison to IfSAR and SRTM but using NED, flood predictions overestimate flood extent in comparison to all other DEMs including LiDAR, the most accurate. This study highlights utility in SRTM as a global source of terrain data for flood modeling.     

How does co-registration affect geomorphic change estimates in multi-temporal surveys?

ABSTRACT

High-Resolution Topography (HRT) data sets are becoming increasingly available, improving our ability and opportunities to monitor geomorphic changes through multi-temporal Digital Terrain Models (DTMs). The use of repeated topographic surveys enables inferring the sediment dynamics of hazardous geomorphic processes such as floods, debris flows, and landslides, and allows us to derive important information on the risks often associated with these processes. The topographic surveying platforms, georeferencing systems, and processing tools have seen important developments in the last two decades, in particular Light Detection And Ranging (LiDAR) technology used in Airborne Laser Scanning (ALS) and Terrestrial Laser Scanning (TLS). Moreover, HRT data, produced through these techniques, changed a lot in terms of point cloud density, accuracy and precision over time. Therefore, old “legacy” data sets and recent surveys can often show comparison problems, especially when multi-temporal data are not homogeneous in terms of quality and uncertainties. In this context, data co-registration should be used to guarantee the coherence among multi-temporal surveys, minimizing, on stable areas, the distance between corresponding points acquired at different epochs. Although several studies highlight that this process is fundamental to properly compare multi-temporal DTMs, it is often not addressed in LiDAR post-processing workflows. In this paper we focus on the alignment of multi-temporal surveys in a topographically complex and rugged environment as the Moscardo debris-flow catchment (Eastern Italian Alps), testing various co-registration methods to align multi-temporal ALS point clouds (i.e. years 2003, 2009 and 2013) and the derived DTMs. In particular, we tested the pairwise registration with manual correspondences, the Iterative Closest Point (ICP) algorithm and a mathematical model that allows aligning simultaneously a generic number of point clouds, the so-called Generalized Procrustes Analysis (GPA), also in its GPA-ICP variant. Then, to correct the possible small inaccuracies generated from the gridding interpolation process, a custom-developed DTM co-registration tool (GRD-CoReg) was used to align gridded data. Both alignment phases (i.e. at point cloud and DTM level) proved to be fundamental and allowed us to obtain proper and reliable DTMs of Difference (DoDs), useful to quantify the debris mobilized and to detect the spatial and temporal patterns of catchment-scale erosion and deposition. The consistency of DoDs data was verified through the comparison between the erosion estimate of DoDs and the volumes of debris-flow events measured by the monitoring station close to the Moscardo torrent catchment outlet. The GPA-ICP algorithm followed by the GRD-CoReg tool proved to be the most effective solution for improving DoDs results with a decrease of systematic trend due to vertical and horizontal uncertainties between surveys, especially at steep slopes. The net volume difference (i.e. the sediment output from the catchment) of the 2003–2013 period changed from 3,237,896 m³ to 135,902 m³ in DoDs obtained from not co-registered and co-registered DTMs. The volume of debris flows measured at the catchment outlet during the same time interval amounts to 169,660 m³. The comparison with debris-flow volume measures at the monitoring station shows, therefore, that the DTMs obtained from the co-registration processes generate more reliable DoDs than those obtained from the raw DTMs (without the alignment).     

DTM94大气模型及其应用

简述了ＤＴＭ９４ 大气模型, 并以其旧版本ＤＴＭ７８ 为对照进行了初步考察和分析, 其中给出了两种模型的大气密度随地磁指数ｋｐ 和太阳辐射流量（Ｓｏｌａｒ Ｒａｄｉｏ Ｆｌｕｘ） 变化的情况, 并对２０ｄ （ 天） 弧长Ａｊｉｓａｉ 卫星的全球ＳＬＲ观测资料进行处理, 结果表明ＤＴＮ９４ 对近地卫星Ａｊｉｓａｉ 的精密定轨是十分有利的。     

Unstructured mesh generation and landcover-based resistance for hydrodynamic modeling of urban flooding

Urban flood inundation modeling with a hydrodynamic flow solver is addressed in this paper, focusing on strategies to effectively integrate geospatial data for unstructured mesh generation, building representation and flow resistance parameterization. Data considered include Light Detection and Ranging (LiDAR) terrain height surveys, aerial imagery and vector datasets such as building footprint polygons. First, a unstructured mesh-generation technique we term the building-hole method (BH) is developed whereby building footprint data define interior domain boundaries or mesh holes. A wall boundary condition depicts the impact of buildings on flood hydrodynamics. BH provides an alternative to the more commonly used method of raising terrain heights where buildings coincide with the mesh. We term this the building-block method (BB). Application of BH and BB to a flooding site in Glasgow, Scotland identifies a number of tradeoffs to consider at resolutions ranging from 1 to 5 m. At fine resolution, BH is shown to be similarly accurate but execute faster than BB. And at coarse resolution, BH is shown to preserve the geometry of buildings and maintain better accuracy than BB, but requires a longer run time. Meshes that ignore buildings completely (no-building method or NB) also support surprisingly good flood inundation predictions at coarse resolution compared to BH and BB. NB also supports faster execution times than BH at coarse resolution because the latter uses localized refinements that mandate a greater number of computational cells. However, with mesh refinement, NB converges to a different (and presumably less-accurate) solution compared to BH and BB. Using the same test conditions, Hunter et al. [Hunter NM, Bates PD, Neelz S, Pender G, Villanueva I, Wright NG, Liang D, et al. Benchmarking 2D hydraulic models for urban flood simulations. ICE J Water Manage 2008;161(1):13–30] compared the performance of dynamic-wave and diffusive-wave models and reported that diffusive-wave models under-predicted the longitudinal penetration of the flood zone due to important inertial effects. Here, we find that a relatively coarse-mesh implementation of a dynamic-wave model suffers from the same drawback because of numerical diffusion. This shows that whether diffusion is achieved through the mathematics or numerics, the effect on flood extent is similar. Finally, several methods of distributing resistance parameters (e.g., Manning n) across the Glasgow site were evaluated including methods that utilize aerial imagery-based landcover classification data, MasterMap^® landcover classification data and LiDAR-based feature height data (e.g., height of shrubs or hedges). Results show that landcover data is more important than feature height data in this urban site, that shadows in aerial imagery can cause errors in landcover classification which degrade flood predictions, and that aerial imagery offers a more detailed mapping of trees and bushes than MasterMap^® which can locally impact depth predictions but has little impact on flood extent.     

空间信息系统原理(三)数字高程模型及其数据结构

简要介绍了DTM的提出和应用情况,并对数字地面模型(DTM)、数字高程模型(DEM或DHM)、数字地形模型(DGM)等三个概念进行了论述,说明了它们的区别和联系,详细论述了离散点、不规则三角形、等高线、断面线和规则格网等几种数字高程模型的数据结构,并对不同数据结构的数字地面模型的特征进行了比较。