Generating GPS satellite fractional cycle bias for ambiguity-fixed precise point positioning

With the development of precise point positioning (PPP), the School of Geodesy and Geomatics (SGG) at Wuhan University is now routinely producing GPS satellite fractional cycle bias (FCB) products with open access for worldwide PPP users to conduct ambiguity-fixed PPP solution. We provide a brief theoretical background of PPP and present the strategies and models to compute the FCB products. The practical realization of the two-step (wide-lane and narrow-lane) FCB estimation scheme is described in detail. With GPS measurements taken in various situations, i.e., static, dynamic, and on low earth orbit (LEO) satellites, the quality of FCB estimation and the effectiveness of PPP ambiguity resolution (AR) are evaluated. The comparison with CNES FCBs indicated that our FCBs had a good consistency with the CNES ones. For wide-lane FCB, almost all the differences of the two products were within ±0.05 cycles. For narrow-lane FCB, 87.8 % of the differences were located between ±0.05 cycles, and 97.4 % of them were located between ±0.075 cycles. The experimental results showed that, compared with conventional ambiguity-float PPP, the averaged position RMS of static PPP can be improved from (3.6, 1.4, 3.6) to (2.0, 1.0, 2.7) centimeters for ambiguity-fixed PPP. The average accuracy improvement in the east, north, and up components reached 44.4, 28.6, and 25.0 %, respectively. A kinematic, ambiguity-fixed PPP test with observation of 80 min achieved a position accuracy of better than 5 cm at the one-sigma level in all three coordinate components. Compared with the results of ambiguity-float, kinematic PPP, the positioning biases of ambiguity-fixed PPP were improved by about 78.2, 20.8, and 65.1 % in east, north, and up. The RMS of LEO PPP test was improved by about 23.0, 37.0, and 43.0 % for GRACE-A and GRACE-B in radial, tangential, and normal directions when AR was applied to the same data set. These results demonstrated that the SGG FCB products can be produced with high quality for users anywhere around the world to carry out ambiguity-fixed PPP solutions.     

The use of WorldView-2 satellite imagery to model urban drainage system with low impact development (LID) Techniques

Within a wide range of best management practices for stormwater management in urban areas, there has been an increasing interest in source control measures. Source controls such as low-impact development (LID) techniques are potentially attractive as retrofit options for older developed areas that lack available land to implement conventional measures such as stormwater management ponds. Hence, distributed urban drainage models requiring detailed representation of developed drainage areas should be developed to accurately estimate the benefits that LIDs may provide. This study (1) presents a two-stage classification process on a high-resolution WorldView-2 image, and (2) demonstrates how to use the extracted land cover information in the subsequent hydrologic modelling and assessment of different LIDs’ performance. The proposed two-stage classification method achieved an overall accuracy of 80.6%, whereas a traditional pixel-based achieved 68.4% in classifying the same urban area into six land cover classes. From the classification results, the hydrologic properties of micro-subcatchments were imported in the United States Environmental Protection Agency Storm Water Management Model to assess the performance of LIDs. A reduction of run-off volume 18.2% and 37.1% was found with the implementation of porous pavement and bioretention, respectively, in a typical low-rise residential area located in the city of San Clemente, California, US. The study demonstrates the use of high-resolution remote sensing image to aid in evaluating LID retrofit options, and thus benefits in situations where detailed drainage area information is not available.     

Classifying Human Activity Patterns from Smartphone Collected GPS data: A Fuzzy Classification and Aggregation Approach

Smartphones have emerged as a promising type of equipment for monitoring human activities in environmental health studies. However, degraded location accuracy and inconsistency of smartphone‐measured GPS data have limited its effectiveness for classifying human activity patterns. This study proposes a fuzzy classification scheme for differentiating human activity patterns from smartphone‐collected GPS data. Specifically, a fuzzy logic reasoning was adopted to overcome the influence of location uncertainty by estimating the probability of different activity types for single GPS points. Based on that approach, a segment aggregation method was developed to infer activity patterns, while adjusting for uncertainties of point attributes. Validations of the proposed methods were carried out based on a convenient sample of three subjects with different types of smartphones. The results indicate desirable accuracy (e.g. up to 96% in activity identification) using of this method. Two examples are provided in the Appendix to illustrate how the proposed methods could be applied in environmental health studies. Researchers could tailor this scheme to fit a variety of research topics.     

TS50全站仪配合多测回测角软件在地铁基坑监测中的应用

TS50全站仪配合徕卡多测回测角软件能够自动瞄准、自动观测,具有高智能、高精度的特点,从而在地铁基坑监测领域得到广泛的应用.文章结合济南市轨道交通R1线前大彦站基坑监测项目实践,对测量方法、数据处理、监测效果等方面进行论述.实际应用表明:基于TS50全站仪的多测回测角软件能提供及时可靠的监测数据,科学指导基坑施工,保证...     

基于偏移四叉树投票的“大尺寸”点状符号多尺度无压盖可视化

为解决Web 2.0环境下点状符号地图混搭中的制图问题,本文研究并实现了一种可100%避免压盖的"大尺寸"点符号高效可视化方法。该方法的核心思想是四叉树网格单选,采用网格平移对多次单选结果投票来计算符号在各缩放级别的显著性等级,可解决符号在相邻网格的空间冲突。该过程不需要显式探测冲突,因而处理效率极高。随着地图放大,重要性较低的符号也逐级显现,实现了语义层次的多尺度表达。针对符号和网格大小比率关系、有效网格平移方案及图面利用率不足问题提出两种扩展:格网增选和多级符号叠加。对方法的可行性进行了试验验证,并分析了该方法在用户查询条件改变下的稳定性和不同数据量下的伸缩性(非优化实现可达到105量级数据的亚秒级处理)。     

基于SRTM-ASTER融合数据的河网提取

在分析SRTM、ASTER GDEM数据源基本特征的基础上,融合了一种直接面向河网提取的数据——SRTM-ASTER,利用SRTM数据较高的垂直分辨率修正ASTER GDEM,弥补了SRTM水平分辨率上的不足;引入Landsat-8遥感卫星数据提取真实河网,分析在有(无)河网辅助条件下融合数据的提取效果,并以总长度、支...     

GIS的矿区土壤重金属污染评价及空间分布

针对土壤中的重金属含量超标会对人体健康造成极大危害的问题,为检测矿区土壤重金属含量超标状况及空间分布特征,以江西省信丰县4个矿区为例,在28个点位的20~60cm处测定Hg、Cd、As、Cu、Pb、Ni的含量,采用地统计学和地理信息系统相结合的方法进行分析。结果表明,单因子污染指数显示Pb的污染程度最大,污染程度为中度污染;插值分析图显示Cu以西南方向的虎山矿区含量较高,Ni以西南方向的虎山矿区和北部的赤岗矿区含量较高,Pb的污染区域贯穿于整个分布区;重金属含量随深度的增加无明显变化。结合在污染修复方面的经验,建议通过植物修复技术、物理与化学方法进行污染治理和修复。     

历史时期(1765-1980年)西藏水旱雪灾规律的探讨

本文利用近千份藏文灾情历史档案和清政府驻西藏办事大臣有泰的日记和联豫的奏稿,以及西藏气象观测记录(1951—1980年),对西藏高原历史时期的水、旱、雪灾作了初步探讨,主要结论如下: 1.近百年来藏南的水旱灾是交替出现的,有三个多水期(1883—1906,1916—1934,1947—1962)和三个干旱期(1907—1915,1935—1946,1963—1980)。干旱期有逐渐加长的趋势。 2.西藏除了有变干的总趋势外,水旱灾还有明显的3.8—2.6年的周期。这与普遍存在的“准两年脉动”周期大体上一致。 3.藏南农区的降水量即使高于常年的一倍也不易形成洪水泛滥和大面积的内涝,但年降水量比常年少100mm以上就可造成严重干旱,农业生产大幅度减产。 4.造成西藏干旱的原因很多也很复杂,除高原气候近期有变干暖的总趋势外,近20年来水浇地和有效灌溉面积的扩大,以及农作物的种植比例不适当,象过份扩大冬小麦种植面积等亦会造成农业干旱。 5.19世纪以来,藏北高原共发生15次严重大雪灾,其中尤以1828—1829年,1927—1928年,1887—1888年三次雪灾最为严重。但近50年,发生的雪灾较过去有所减轻。不过,1967—1968年雪灾也较为严重。     

基于DEM的彩色地貌晕渲的制作

本文围绕计算机环境下数字地貌晕渲实现的理论与技术展开了探讨,论述了数字地貌晕渲的相关理论,介绍了数字地貌彩色晕渲色彩设计的基本方法,对数字地貌晕渲的技术过程进行了深入研究。     

树状河系自动绘制的结构化实现

计算机的出现和信息论的发展,使制图自动化在技术上和理论上得到了迅速发展。然而由于目前软件系统还不能代替制图工作者的创造性思维,在制图过程中仍需要大量的专业知识以及人机对话,尤其在符号化大面积的河流时,重复的河流宽度手工设置过程大大增加了制图人员的工作量。提出河系绘制结构化实现的思想,将由多条单线河流组成的树状河系通过其结构实现自动符号化,希望能减轻制图人员的负担。