1,721,081 research outputs found
SPATIO-TEXTUAL TRAJECTORIES: MODELS AND APPLICATIONS
Full text linkL'argomento della Tesi riguarda la gestione e analisi di dati di mobilità . La pervasività delle tecnologie per la geolocalizzazione, sensori e reti di comunicazione, ha reso possibile l’acquisizione di grandi quantità di dati sul movimento di oggetti, da cui la necessità di tecniche che consentano l’organizzazione e l’accesso efficiente ai dati. Una classe importante di dati di mobilità è costituita dalle traiettorie spaziali. Una traiettoria spaziale descrive il movimento continuo di un oggetto in uno spazio di riferimento, e.g. spazio Euclideo, tramite un sequenza di punti campione, temporalmente annotati e ordinati. Le traiettorie spaziali possono descrivere ad esempio il movimento di veicoli, individui o animali, equippaggiati con un ricevitore GPS.
Le traiettorie spaziali, tuttavia, descrivono il movimento unicamente in termini di posizione mentre non permettono di descrivere l’evoluzione nel tempo del contesto piu’ ampio in cui il movimento ha luogo. I dati di contesto possono essere acquisiti direttamente dall’ambiente, ad esempio tramite l’uso di sensori, oppure ottenuti dall’applicazione di tecniche analitiche. In questo senso le traiettorie spaziali difettano di espressività. Un passo importante nella direzione di modelli piu’ ricchi ed espressivi è costituito dal modello delle traiettorie ‘simboliche’. Questo modello permette di descrivere sequenze di attività tramite etichette, ognuna annotata da un intervallo temporale. L’aspetto piu’ innovativo del modello riguarda lo sviluppo di un linguaggio di interrogazione basato su pattern matching. Anche questa soluzione presenta tuttavia importanti limiti perchè le traiettorie simboliche sono completamente ortogonali a quelle spaziali e quindi ignorano il dato di posizione.
Questa Tesi affronta il problema della integrazione della dimensione spaziale con quella simbolica per un accesso efficiente a database di traiettorie denominate spazio-testuali. Il contributo piu’ significativo e’ un sistema per la indicizzazione di traiettorie spazio-testuali a supporto delle interrogazioni di tipo Sequenced Queries, ossia queries espresse come sequenze ordinate di queries elementari spazio-testo. Il sistema si chiama IRWI, e’ un sistema di indici ibrido che combina un indice basato su R-tree per la indicizzazione dei segmenti di traiettoria con un indice basato su inverted file per la indicizzazione dela componente testuale. Il processamento delle query viene effettuato in parallelo valutando le singole queries della sequenza e poi alla fine ricomponendo le sequenze.
Un secondo aspetto trattato nella tesi riguarda lo studio di tecniche per l’analisi dei dati di movimento. L’obiettivo è quello di estrarre pattern comportamentali dalle traiettorie spaziali per poi rappresentarli in termini di traiettorie spazio-testuali. Il contributo più significativo è la definizione di un algoritmo di segmentazione delle traiettorie che sfrutta tecniche di clustering con un nuovo modello di noise.
In ultimo, è stato realizzato un caso di studio che illustra e riassume la metodologia proposta per l’ analisi e rappresentazione del movimento. La tecnica di segmentazione di cui sopra è stata utilizzata per l’estrazione da traiettorie spaziali del comportamento migratorio di animali equipaggiati con collari GPS. Questa conoscenza è stata poi espressa in termini di traiettorie spazio-testuali. Complessivamente, i risultati della ricerca hanno dato luogo a diverse pubblicazioni in riviste e a conferenze.The Thesis concerns the management and analysis of mobility data. The pervasiveness of geo-positioning technologies, sensors and communication networks has led to the collection of large amounts of data on the movement of objects. A major issue is thus how to effectively organize and access such a data. An important category of mobility data is that of spatial trajectories. A spatial trajectory describes the continuous movement of an object in a reference space, e.g. the Euclidean plane, through a set of temporally annotated and ordered sample points. Spatial trajectories can represent the movement of vehicles, people and animals, for example equipped with a GPS receiver.
Yet, spatial trajectories can represent the movement exclusively in terms of locations, thus the evolution of the context in which the movement takes place is ignored. In general, the contextual data can be acquired directly from the environment, for example through the use of sensors, or be the result of an analytical process. In this sense, spatial trajectories lack expressivity. An important step towards the specification of richer and more expressive data models, is the symbolic trajectories data model. This model allows for the representation of sequences of activities (or labels), each annotated with a time period. A major novelty of the model is the query language that is based on pattern matching. Nevertheless also this solution presents important limitations because the notion of symbolic trajectory is orthogonal to that of spatial trajectory and thus does not include any location information.
The Thesis addresses the problem of integrating the spatial dimension with the symbolic one, providing as well a mechanism enabling the efficient access to a database of spatio-textual trajectories. The major contribution of this research is the proposal of a framework for the indexing of spatio-textual trajectories The goal of the index is to support the processing of queries taking the form of Sequenced queries, that is complex queries expressed as sequences of ordered simple spatio-textual queries. The index is called IRWI. The system is hybrid in that it combines an R-tree for the indexing of spatial trajectory segments with inverted files for the indexing of the textual part. A Sequenced query is next processed in parallel, evaluating first every single query of the sequence, and finally analyzing and recomposing the sequence.
A related though different topic of the Thesis regards the study of techniques for mobility data analysis. The objective is to extract behavioral patterns from spatial trajectories and next represent them in terms of spatio-textual trajectories. The major contribution is the definition of an algorithm for the segmentation of the trajectories based on clustering and relying on a novel model of noise. Finally, a case study illustrates and summarizes the methodology proposed for the analysis and representation of mobility data. Specifically the above segmentation technique is used for the extraction of the migratory behavior of a group of animals equipped with GPS collars. Next such a knowledge is encoded in terms of spatio-textual trajectories.
The results of this research, spanning data representation and analysis, have been presented in conferences and journals
Efficient access to temporally overlaying spatial and textual trajectories
No full textIn this paper we deal with the problem of querying heterogeneous trajectory data consisting of spatial and textual trajectories defined over continuous temporal domains. In contrast with spatial trajectories, which describe the continuous movement in space of an individual, the textual trajectories of concern in this work describe the individual’s step-wise changing behavior, such as the transportation means used and the activities performed in a time period. Accessing large datasets of temporally aligned spatial and textual trajectories can provide valuable information on where certain behaviors take place. In this paper we present a novel index framework, called IRWI, for the efficient processing of queries on aligned spatio-textual trajectories formulated as sequences of ordered spatio-textual range queries q = q1, .., qn (sequenced queries). IRWI consists of a hybrid, spatial and textual, index data structure, enriched with a number of features that facilitate the early pruning of trajectories during the concurrent evaluation of the sub-queries q1, .., qn. As a result, the IRWI tree can be traversed only once. The experiments, conducted on both synthetic and real datasets, show a gain in performance with respect to state-of-the art techniques that increases significantly with the length of the query sequence
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Symbolic trajectories and application challenges
No full textDescribing the location history of moving objects exclusively in geometric terms is no longer sufficient, whereas more expressive data models capturing the complexity and heterogeneity of movement data are needed. Following this trend, the data model of symbolic trajectories has been recently proposed for the representation of content-rich trajectories in databases. The model provides a simple notation and a powerful and fully operational pattern-based query language for trajectory matching and rewriting. In this paper, we overview the key features of the model and sketch two applications cases, the former regarding the integration of heterogeneous mobility data (GPS and transportation modes), the latter the representation of migration patterns in animal ecology. The goal is to show the flexibility of the model and, at the same time, to prospect possible directions of research
Discovering gatherings based on individual mobility patterns: challenges and direction
No full textGiven a database of spatial trajectories reporting the movement of a set of objects in a time frame, the problem is to discover the groups of objects that stay in close proximity within a geographical area for a significant time. To deal with the problem, techniques for the discovery of collective patterns, e.g. the meeting pattern, have been proposed. Such techniques, however, impose stringent constraints on the object movement. In this paper we investigate a generalized, flexible approach that builds on the idea of expressing the collective pattern as sum of individual behaviors. We present a technique called k-Gathering for the discovery of gatherings of at least k objects, which leverages a recent method for the discovery of stop-and-move patterns. The experiments, conducted on both synthetic and real data, show that the direction is promising and that the approach can be effective also on low sampling rate trajectories
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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