Extracting activity patterns from taxi trajectory data: a two-layer framework using spatio-temporal clustering,Bayesian probability and Monte Carlo simulation

ABSTRACT

Global positioning system (GPS) data generated from taxi trips is a valuable source of information that offers an insight into travel behaviours of urban populations with high spatio-temporal resolution. However, in its raw form, GPS taxi data does not offer information on the purpose (or intended activity) of travel. In this context, to enhance the utility of taxi GPS data sets, we propose a two-layer framework to identify the related activities of each taxi trip automatically and estimate the return trips and successive activities after the trip, by using geographic point-of-interest (POI) data and a combination of spatio-temporal clustering, Bayesian inference and Monte Carlo simulation. Two million taxi trips in New York, the United States of America, and ten million taxi trips in Shenzhen, China, are used as inputs for the two-layer framework. To validate each layer of the framework, we collect 6,003 trip diaries in New York and 712 questionnaire surveys in Shenzhen. The results show that the first layer of the framework performs better than comparable methods published in the literature, while the second layer has high accuracy when inferring return trips.     

Uncertainties in the geographic context of health behaviors: a study of substance users’ exposure to psychosocial stress using GPS data

ABSTRACT

This study examined how contextual areas defined and operationalized differently may lead to different exposure estimates. Substance users’ exposures to environmental stress (in terms of two variables: community social economic status and crime) were assessed from global positioning systems (GPS) data. Participants were 47 outpatients with substance use disorders admitted for methadone maintenance at a research clinic in Baltimore, Maryland. From 35.2 million GPS tracking points, we compared 7 different methods for defining activity space. The different methods yielded different exposure estimates, which would lead to different conclusions in studies using only one method. These results have important implications for future research on the effect of contextual influences on health behaviors and outcomes: whether a study observes any significant influence of an environmental factor on health may depend on what contextual units are used to assess individual exposure.     

Geography Current

Abstract

This article explains a professional development experience of fifth to twelfth grade teachers in using geographic information systems (GIS) and global positioning systems (GPS) technologies to enhance classroom teaching and learning environments. A key challenge faced by the developers was whether teachers would value the technology tools enough to warrant the time necessary to develop the skills for productive use of the technology. Based on five years' experience, researchers identified seven key components and elaborated on them with examples and related processes.     

Geographic Information Technologies and the History of a Shoreline: The Case of Chapter 91 in New Bedford,Massachusetts

Abstract

This article discusses the use of geographic information technologies to assist researchers in the exploration of historical databases. The case study is a pilot project in which we used geographic information systems (GIS) and global positioning systems (GPS) to study the history of the Public Waterfront Act (the “Chapter 91” program), a policy designed to balance private property rights, public interest, and environmental protection in the Massachusetts tidelands. The issues discussed range from the role of GIS in society and its limitations as a representational tool to the ability of current GIS to deal with historical data and to manage temporal attributes.

     

Map-matching algorithm for large-scale low-frequency floating car data

Large-scale global positioning system (GPS) positioning information of floating cars has been recognised as a major data source for many transportation applications. Mapping large-scale low-frequency floating car data (FCD) onto the road network is very challenging for traditional map-matching (MM) algorithms developed for in-vehicle navigation. In this paper, a multi-criteria dynamic programming map-matching (MDP-MM) algorithm is proposed for online matching FCD. In the proposed MDP-MM algorithm, the MDP technique is used to minimise the number of candidate routes maintained at each GPS point, while guaranteeing to determine the best matching route. In addition, several useful techniques are developed to improve running time of the shortest path calculation in the MM process. Case studies based on real FCD demonstrate the accuracy and computational performance of the MDP-MM algorithm. Results indicated that the MDP-MM algorithm is competitive with existing algorithms in both accuracy and computational performance.     

A novel methodology for identifying environmental exposures using GPS data

Aim: While studies using global positioning systems (GPS) have the potential to refine measures of exposure to the neighbourhood environment in health research, one limitation is that they do not typically identify time spent undertaking journeys in motorised vehicles when contact with the environment is reduced. This paper presents and tests a novel methodology to explore the impact of this concern.

Methods: Using a case study of exposure assessment to food environments, an unsupervised computational algorithm is employed in order to infer two travel modes: motorised and non-motorised, on the basis of which trips were extracted. Additional criteria are imposed in order to improve robustness of the algorithm.

Results: After removing noise in the GPS data and motorised vehicle journeys, 82.43% of the initial GPS points remained. In addition, after comparing a sub-sample of trips classified visually of motorised, non-motorised and mixed mode trips with the algorithm classifications, it was found that there was an agreement of 88%. The measures of exposure to the food environment calculated before and after algorithm classification were strongly correlated.

Conclusion: Identifying non-motorised exposures to the food environment makes little difference to exposure estimates in urban children but might be important for adults or rural populations who spend more time in motorised vehicles.     

北斗系统在远洋及南极地区的定位性能分析

利用北斗/GPS双系统四频率接收机,于2011年10月至2012年4月采集了中国第28次南极考察沿线的北斗和GPS实测数据,跨度北至中国天津,南至南极内陆昆仑站。从C/A码伪距的信噪比和多路径、可见卫星数、PDOP值、标准定位的精度等方面对比分析了北斗和GPS系统在航线上不同区域,尤其是在远洋及南极地区不同运动状态下的定位效果。结果表明现阶段的北斗系统信号质量总体上与GPS相当;在45°以内的中低纬度地区,动态定位精度与GPS相当,水平和高程方向分别可达10 m和20 m左右,而静态定位水平方向精度为米级,与GPS相当,高程方向10 m左右,较GPS略差;在中高纬地区,由于可见卫星数较少、卫星分布较差,定位精度较差或无法定位。     

GPS全球定位系统及其在地学遥感中的应用价值

本文阐述了ＧＰＳ全球定位系统的定位特点、结构性能、观测原理与工作方式，在此基础上分析说明了ＧＰＳ在地学遥感中的应用价值：（１）野外调查中的定位监测与测量；（２）遥感影象定位校正；（３）ＧＩＳ地学编码；（４）全球环境信息定位。     

中国东北漫岗黑土区切沟侵蚀发育特征

In this study, short-term gully retreat was monitored from the active gullies selected in representative black soil area, using differential global positioning system (GPS). With the support of geographic information system (GIS), multi-temporal digital elevation models (DEM) were constructed from the data collected by GPS and used for further analysis. Based on the analysis of multi-temporal DEM, we discussed the erosion-deposition characteristics within gully and a developing model for black soil gully area of Northeast China was proposed. The results are: (1) The analysis of the monitored gully data in 2004 indicated that the retreat of gully head reached more than 10 m, gully area extended 170–400 m², net gully eroded volume 220–320 m³, and gully erosion modulus 2200–4800 t·km⁻²·a⁻¹. (2) Compared with the mature gully the initial gully grows rapidly, and its erosion parameters are relatively large. The erosion parameters have not only to do with flow energy, but also with the growth phase. (3) There are significant seasonal differences in gully erosion parameters. The extension of gully area and width dominates in winter and spring without marked net erosion while changes mainly occur in gully head and net erosion in rainy season. (4) It is remarkable for freeze-thaw erosion in the black soil area of NE China. The gully wall of SG2 extended 0.45 m under freeze-thaw effect in 2004, and the distance of gully head retreated maximally 6.4 m. (5) Due to freeze-thaw action and snowmelt, gully is primarily in the interior adjustment process in winter and early spring. There are much more depositions compared with that during rainy season, which can almost happen throughout the gully, while erosion mostly occurs near head, esp. for gullies having a relatively long history of development. On the other hand, the process of energy exchange with exterior dominates in rainy season. It is considered that this cyclic process is an important mechanism for gully growth in high latitude or/and high attitude regions. Foundation: Key Project for National Natural Science Foundation of China, No.40235056; The Ph.D. Programs Foundation of Ministry of Education of China, No.20030027015; China Postdoctoral Science Foundation, No.20070410482; Doctoral Foundation of University of Jinan, No.B0620; National Natural Science Foundation of China, No.40672158; Key Subject Foundation Supported by Shandong Province Author: Hu Gang (1976–), Ph.D and Associate Professor, specialized in soil erosion, environmental evolution and regional planning.     

Why GPS makes distances bigger than they are

Global navigation satellite systems such as the Global Positioning System (GPS) is one of the most important sensors for movement analysis. GPS is widely used to record the trajectories of vehicles, animals and human beings. However, all GPS movement data are affected by both measurement and interpolation errors. In this article we show that measurement error causes a systematic bias in distances recorded with a GPS; the distance between two points recorded with a GPS is – on average – bigger than the true distance between these points. This systematic ‘overestimation of distance’ becomes relevant if the influence of interpolation error can be neglected, which in practice is the case for movement sampled at high frequencies. We provide a mathematical explanation of this phenomenon and illustrate that it functionally depends on the autocorrelation of GPS measurement error (C). We argue that C can be interpreted as a quality measure for movement data recorded with a GPS. If there is a strong autocorrelation between any two consecutive position estimates, they have very similar error. This error cancels out when average speed, distance or direction is calculated along the trajectory. Based on our theoretical findings we introduce a novel approach to determine C in real-world GPS movement data sampled at high frequencies. We apply our approach to pedestrian trajectories and car trajectories. We found that the measurement error in the data was strongly spatially and temporally autocorrelated and give a quality estimate of the data. Most importantly, our findings are not limited to GPS alone. The systematic bias and its implications are bound to occur in any movement data collected with absolute positioning if interpolation error can be neglected.     

Critical Thinking in Physical Geography: Linking Concepts of Content and Applicability

Abstract

This research examines the effectiveness of problem-based learning (PBL) in an online geographic information systems (GIS) course for adult learners. Adults expect to take greater control of their learning and tend to be self-directed. Students are encouraged to take an active role as problem-solvers in PBL courses. The effectiveness of PBL in face-to-face courses has been shown. Effectiveness in online courses is demonstrated in this study. The performance and experiences of students and instructor in two online courses are compared. The results show that PBL can be more engaging than traditional methods, making PBL-GIS a good choice for an online adult audience.     

Changes of Snow Cover Thickness Measured by Conventional Mass Balance Methods and by Global Positioning System Surveying

The most labour-intensive and time-consuming part of many mass balance programmes is the acquisition of snow depth data. The standard technique, which involves probing the snow cover at intervals along a series of profiles, generally by an individual on skis, may involve more than 300 discrete measurements along a total of more than 20 km of profiling at a single glacier. Kinematic surveying with a global positioning system (GPS) in differential mode provides much more information about changes of glacier surface level and snow thickness between surveys. The positions of a large number of points can be fixed in a relatively short time by GPS surveying, and the technique is usable in adverse weather conditions. With real-time kinematic GPS surveying, it is possible to return to the same positions (longitude, latitude) during successive field programmes, and a previously followed route can be retraced precisely. GPS surveying facilitates the production of accurate glacier maps for mass balance programmes. Data obtained by snow depth probing and GPS surveying in 1995 at Austre Okstindbreen, the largest glacier of the Okstindan area, Norway (66°N), indicate that repeated GPS surveys are likely to provide a large amount of information on withinyear and between-year changes of surface topography and are not subject to the errors in mass balance calculations which arise from probing snow depths along selected profiles. Kinematic GPS surveying of several glaciers within an area would overcome the difficulties arising when mass balance studies are confined to a single glacier within a particular area.     

北斗与GPS 集成的高精度定位及桥梁监测分析

GNSS 可提供全天候的实时高精度定位结果,是形变监测的主要方法之一,但卫星信号极易受到周围环境如建筑物、人群等遮挡的影响,而相当数量的基础设施修建于山谷、城市建筑群、车流密集等区域。文章研究在遮挡环境下卫星定位应用于形变监测领域的可行性问题。因中国北斗系统在星座布设上与GPS 不同,采用GEO、IGSO 及MEO 三种不同轨道类型形成高轨卫星与中轨卫星的结合,不同的轨道类型有不同的特点,对定位精度的影响也不尽相同。以广州大桥为例,通过对北斗/GPS 集成高精度定位的研究,并针对北斗卫星的不同轨道类型,利用GEO、IGSO 及MEO 之间的不同组合进行定位,分析其在形变监测的可行性。研究表明：北斗/GPS 集成高精度定位精度可达毫米级,能够满足形变监测的需求;同时,北斗/GPS 集成定位可增加可视卫星数,改善卫星的几何分布,而北斗系统的高轨卫星高度角较高,特别是GEO 和IGSO 两种高轨卫星的组合仍能达到1 cm 的定位精度,大大提高了山谷、高边坡、城市峡谷等遮挡较为严重区域中卫星高精度定位的可行性,有利于形成全天候自动化实时高精度的形变监测与灾害监测安全预警系统。     

Designing a personal guidance system to aid navigation without sight: progress on the GIS component

Abstract

This paper reports on progress towards the development of a personal guidance system for a blind navigator. The system is being developed in light of the unique problems facing the blind traveller, which are discussed first. The system consists of four modules: a locator unit, a detailed spatial database, an algorithm for path selection and a user interface. Suggestions are offered for alternative ways of handling the locator problem using a global positioning system or a database query system. The nature of the geographic information system (GIS) to be used as a host for the database is discussed, followed by brief statement of the criteria for choosing an algorithm for path selection and suggestions on types of user interfaces best suited to the system. A report on a pilot-study GIS is presented. Special problems of database and system design are highlighted.     

基于GPS的北极冰川表面地形测量与制图

自2005年以来,我国科考队员利用双频GPS在北极黄河站附近的Austre Lovénbreen 和 Pedersenbreen两条冰川上每年一次开展高精度的冰川运动观测,获取了冰川表面监测标杆的精确位置和运动速度。2009年4月,我国考察队员在这两条冰川上开展了密集的GPS点位数据采集,藉此开展北极两条冰川的冰面地形测量。在分析单频GPS动态单点定位数据用于冰面地形测量的可行性基础上,经过平差计算获得了两条冰川的冰面地形数据,进而生成冰面DEM和等高线,制作冰面地形图。经与高精度控制点比较,冰面DEM高程的误差为0.78m,在冰川季节性高程波动和年消融的变化范围之内。由于SMART-V1型GPS设备是当前冰川研究工作中应用较多的pulseEKKO型探地雷达配套的一个重要部件,本文的结论对于同类仪器开展冰川测量工作具有参考价值,对基于高密度的GPS动态单点定位测量方法用于冰面地形测量的数据处理具有指导意义。     

THE ARCHIVES OF THE ASSOCIATION OF AMERICAN GEOGRAPHERS

Although numerous courses in geographic information systems (GIS) have been introduced into the geography curriculum over the past few years, there has been remarkably little debate over the issues involved. This paper first discusses the arguments for geography as an appropriate disciplinary setting for GIS teaching at the undergraduate level. This is followed by a discussion of the nature of GIS courses and their appropriate place in the undergraduate geography curriculum. The final section of the paper describes the NCGIA Core Curriculum project and examines its significance in this broader context.     

The Course in Geography in the State Normal School at Salem,Mass.

Abstract

During academic year 1999–2000, the Geography Department at Michigan State University (MSU) launched a program of online geography courses. Since then, four undergraduate online geography courses have been collectively developed and delivered to over 4,500 domestic and international students in association with MSU's Virtual University Design and Technology (vuDAT). The creation of each virtual course has taught valuable lessons concerning course development and administration; interactions with students have shed insight on course structure, content, and instruction. This article addresses development, management, and pedagogic concerns pertinent to the operation of online geography courses and offers suggestions to others who wish to develop online courses. Preliminary evidence on the effectiveness of these courses is also presented.     

Mass Balance Methods on Kongsvegen, Svalbard

On the glacier Kongsvegen (102 km²) in northwest Spitsbergen, Svalbard, traditional mass balance measurements by stake readings and snow surveying have been conducted annually since 1987. In addition, repeated global positioning system (GPS) profiling, shallow core analysis and ground-penetrating radar (GPR) surveying have been applied. The purpose of this paper is to evaluate the input from the different methods, especially the GPS profiling, using the results from the traditional direct method as a reference. The annual flow rate on Kongsvegen is low (2 ? 3 m a^?1), and the emergence velocity is almost negligible. Thus the geometry changes of the glacier, i.e. the change in altitude per distance from the head of the glacier, should reflect the change in net balance of the glacier. The mean annual altitude change from the longitudinal, centreline GPS profiles was compared to the direct stake readings and showed a very good agreement. On Kongsvegen the measured actual ice flux is so low that the mass transfer down-glacier at the mean equlibrium line altitude is less than 10% of what is needed to maintain steady-state geometry. This is clearly shown in the changing altitude profiles. GPS profiling can be used on large glaciers in remote areas to monitor geometry changes, ice flow and net mass balance changes. However, it requires that the centreline profile changes are representative for the area/altitude intervals, i.e. that the accumulation and ablation pattern is evenly distributed. For this purpose the GPR surveying quickly gave the snow distribution variability over long distances. Shallow cores drilled in different altitudes in the accumulation area were analysed to detect radioactive reference layers from the fallout after the Chernobyl accident in 1986, and showed very good agreement to the direct measured net balance. Thus older reference horizons from bomb tests in 1962 could be used to extend the net balance series backwards.     

The Fundamental and the Incidental in Geography

Abstract

An increased use of artistic literature as a resource in both teaching and research is evident in geography. With the growing interest of some scholars in the behavioral aspects of geography, traditional resources are being supplemented with more humanistic, subjective information sources. In historical geography, for example, literature is a particularly suitable resource for studies of man's role in past geographies and of human perception of past environments.¹ Specialists in regional geography and area studies also have shown a growing appreciation of such sources.² A parallel and related development has occurred in geographic education. In many different courses, but particularly in those dealing with regions, literature has become a recommended resource. The purposes of this paper are (1) to review the relevance of the novel as a resource for geographers and (2) to suggest that a current novel, Centennial, by James A. Michener,³ particularly warrants consideration for use in courses on the historical geography of the United States.     

Decisions of the U. S. Board of Geographic Names. June 5, 1900, To January 8, 1902 II

ABSTRACT

Spatial literacy is a new frontier in K–12 education. This article describes a place-based introductory GIS/GPS middle school curriculum unit in which students used measuring tools, GPS units, and My World GIS software to collect physical and spatial data of trees to create a schoolyard tree inventory. Maine students completed “memory maps” of their schoolyards as a pre/post exercise assessment. A statistically significant increase in students’ spatial awareness was documented. A technology-based curriculum can significantly increase students’ spatial awareness especially in a place and context relevant to each student.