Why GPS makes distances bigger than they are

Global Navigation Satellite Systems (GNSS), such as the Global Positioning System (GPS), are among 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 error. 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 is the case for movement sampled at high frequencies. We provide a mathematical explanation of this phenomenon and we 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 strong autocorrelation any two consecutive position estimates have very similar error. This error cancels out when average speed, distance or direction are 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 a set of pedestrian and a set of car trajectories. We find that the measurement error in the data is strongly spatially and temporally autocorrelated and give a quality estimate of the data. Finally, we want to emphasize that all our findings are not limited to GPS alone. The systematic bias and all its implications are bound to occur in any movement data collected with absolute positioning if interpolation error can be neglected.Comment: 17 pages, 8 figures, submitted to IJGI

Military power and the Dutch Republic: war, trade and the balance of power in Europe, 1648-1813

In 1667 the Dutch Republic was at the height of its military and mercantile might. A century and a half later, little of that glory remained as Napoleon wiped the country off the political map. Military Power and the Dutch Republic explores the often overlooked role of the military in the Republic’s remarkable economic rise in the seventeenth century and its subsequent fall. It examines the ways in which the Dutch army and navy were organised and financed, the strategies and tactics that were used, and the operations of military leaders on land and sea. It also investigates methods of recruitment, where and how the army and navy found their troops, how those troops were housed and fed, and how they behaved in battle. And it looks at the various kinds of interaction between the many thousands of ordinary soldiers and sailors and the civilian society whose taxes supported them.Translated by Paul Arblaster and Lee Preed

Evidence-based design: Satellite positioning studies of city centre user groups

In an era of increasing competition between urban cores, positive pedestrian experiences become crucial in attracting visitors and retaining residents. The dichotomy between use and evaluation of public spaces enables the prioritisation of urban design and management interventions in an age of austerity. When spaces are highly used but evaluated as poor, they warrant improvements over other spaces. The results therefore substantiate evidence-based design that achieves significant yet cost-effective improvements of the city centre pedestrian experience. This paper presents an approach to understanding city centre pedestrian behaviour and experiences by deploying global positioning system tracking technology. This technology enables accurate measurement of routing and behaviour in public space. Differences between the use of public spaces in the city centre of Delft by visitors and residents are measured by tracking the behaviour of three user groups. This measured behaviour is supplemented by questionnaire results to understand the evaluation of public spaces in central Delft.UrbanismArchitecture and The Built Environmen