- Institute Directoredit
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. Various relation-based systems, concerned with the representation and processing ofspatial relations, have been developed in numerous application domains. In this thesis weidentify the common concepts underlying qualitative spatial... more
. Various relation-based systems, concerned with the representation and processing ofspatial relations, have been developed in numerous application domains. In this thesis weidentify the common concepts underlying qualitative spatial knowledge representation, wecompare the representational properties of the different systems and we outline thecomputational tasks involved in relation-based spatial information processing. The thesis alsoproposes symbolic spatial indexes, relation-based...
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ABSTRACT Social networks are prevalent on the Internet and become a hot research topic attracting many professionals from a variety of fields. The advances in location-acquisition and mobile communication technologies empower people to... more
ABSTRACT Social networks are prevalent on the Internet and become a hot research topic attracting many professionals from a variety of fields. The advances in location-acquisition and mobile communication technologies empower people to use location data with existing online social networks. The dimension of location helps bridge the gap between the physical world and online social networking services. Furthermore, people in an existing social network can expand their social structure with the new interdependency derived from their locations. This Special Issue focuses on the following important research questions in location based social networks. First, how to acquire quality location data which requires matching GPS locations obtained to map locations. Second, how to capture the correlations between the spatial and temporal aspects of the GPS trajectories. Third, how to mine and query the location data effectively and efficiently. Finally, how to deal with location privacy issues in all these ...
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In this paper, we outline our work on developing a disk-based infrastructure for efficient visualization and graph exploration operations over very large graphs. The proposed platform, called graphVizdb, is based on a novel technique for... more
In this paper, we outline our work on developing a disk-based infrastructure for efficient visualization and graph exploration operations over very large graphs. The proposed platform, called graphVizdb, is based on a novel technique for indexing and storing the graph. Particularly, the graph layout is indexed with a spatial data structure, i.e., an R-tree, and stored in a database. In runtime, user operations are translated into efficient spatial operations (i.e., window queries) in the backend.
Research Interests: Web 2.0, Semantic Web Technologies, Knowledge Management, Visualization, Data Mining, and 61 moreSemantics, Database Systems, Graph Theory, Information Visualization, Data Analysis, XML, Web Technologies, Visual Analytics, Web Applications, Linked Data, Knowledge Discovery in Databases, Web of Data, Semantic Web technology - Ontologies, Semantic Web, Semantic Web Services, Graph Database, Open Data, Databases, Graph/Network Algorithms, Network Visualization, Social Semantic Web, Graph Data Mining, Web 3.0, Data Structures and Algorithms, Database Management Systems, Data Visualization, Data Graph Management, Visual Analysis, SPARQL query processing and optimization, Infographics and data visualization, Data mining, Information Extraction, Deep Web, RDF, OWL, Linked Open Data, Big Data, RDF Databases, Web 2.0 tools, HTML5, CSS3, XML, Web standards, Atom, RSS, RDF, RDFa, OWL, Rdf, Sparql, Query Langauge, Data Structures, Social Graphs, Visualisation, Ontologies, Knowledge representation, Semantic web, Cloud computing, Ontology based data access, Knowlede management, Big Data Analytics, Graph databases, Graph Data Visualization Chart Representation, Data Analysis: Statistical and Graphical, Big Data / Analytics / Data Mining, Hypergraphs, Graphs, Data Mining, Graph Data Structure, Visualization Tools, SPARQL, Graph Visualization, Knowlegde and Knowledge Management, Visualization of Big Data, Big Data Graph Processing, Data Analysis and Visualization, Visual Exploration and Analysis, Web Databases, Graphic Visualization of Data, Visualization of Big Data in Social Networks, and Big Graph Data
In this paper, we outline our ongoing work on diversifying keyword search results on RDF data. Given a keyword query over an RDF graph, we define the problem of diversifying the search results and we present diversification criteria that... more
In this paper, we outline our ongoing work on diversifying keyword search results on RDF data. Given a keyword query over an RDF graph, we define the problem of diversifying the search results and we present diversification criteria that take into consideration both the content and the structure of the results, as well as the underlying RDF/S-OWL schema.
Research Interests: Information Systems, Algorithms, Information Retrieval, Information Technology, Web 2.0, and 86 moreSemantic Web Technologies, Knowledge Management, Natural Language Processing, Machine Learning, Data Mining, Semantics, Database Systems, Diversity, Graph Theory, Digital Media, Data Analysis, Knowledge organization, Policy, Web Technologies, Access Control, Web Development, Data Management, Personalization, Web search, SEO, Web Applications, Linked Data, Knowledge Discovery in Databases, Web Personalization, Search Computing, Information Retreival, Semantic Web technology - Ontologies, Semantic Web, Graph Database, Open Data, Algorithm, Cloud Computing, Online Marketing, Databases, Semantic Search Engine, Query Optimization, Graph/Network Algorithms, Search Engines, Graph Data Mining, Querying the Web of Data, Web 3.0, Database Management Systems, Smart spaces, Diversification, SPARQL query processing and optimization, Database Management Systems and Querry Processing, Cyber Physical Systems, Complex Event Processing, Search Engine Optimization, Ir, Rules, Online Privacy, RDF, OWL, Cyber Security, Semantic Computing, Linked Open Data, Big Data, Online market research, RDF Databases, Search, search engine optimization, search engine results, search engines, search keywords, seo, serps, set, shortcuts, spam, spammers, Spamming, technique, time, web page 15 hours ago, HTML5, CSS3, XML, Web standards, Atom, RSS, RDF, RDFa, OWL, Online Branding, Data Structures, Programming (XSLT, XML, RDF...), Recomender System, Web Promotion, Big Data Analytics, Syntactic and Semantic Knowledge, Ranking, Rif, Big Data / Analytics / Data Mining, Google Analytics, Big Data Visualisation, SPARQL, P2P/Overlay Networks, Graph Algorithm, VoIP/SIP/IMS, Distributed Information Systems, RFID and Sensor Networks, Triple Stores, Content Extraction, SWRL, Web Optimization, Database, and Recomendation Systems
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Multimedia applications usually involve a large number of multimedia objects (texts, images, sounds, etc.). An important issue in this context is the specification of spatial and temporal relationships among these objects. In this paper... more
Multimedia applications usually involve a large number of multimedia objects (texts, images, sounds, etc.). An important issue in this context is the specification of spatial and temporal relationships among these objects. In this paper we define such a model, based on a set of spatial and temporal relationships between objects participating in multimedia applications. Our work exploits existing approaches for spatial and temporal relationships. We extend these relationships in order to cover the specific requirements of multimedia applications and we integrate the results in a uniform framework for spatio-temporal composition representation. Another issue is the efficient handling of queries related to the spatio-temporal relationships among the objects during the authoring process. Such queries may be very costly and appropriate indexing schemes are needed so as to handle them efficiently. We propose efficient such schemes, based on multidimensional (spatial) data structures, for large multimedia applications that involve thousands of objects. Evaluation models of the proposed schemes are also presented, as well as hints for the selection of the most appropriate one, according to the multimedia author's requirements.
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Research Interests: Information Retrieval, Temporal Data Mining, Temporal Databases, Databases, Indexing, and 10 moreDatabase indexing, Predictive models, Query processing, Object Representation, Spatial Data, Boolean Satisfiability, Indexation, Spatiotemporal Databases, Geographic Information Systems, and Database System
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R-trees and related structures, like R+-trees and R*-trees, have been used to answer queries involving topological information about objects represented by their minimum bounding rectangles (MBRs). This paper describes how the R-tree... more
R-trees and related structures, like R+-trees and R*-trees, have been used to answer queries involving topological information about objects represented by their minimum bounding rectangles (MBRs). This paper describes how the R-tree method can be used for the storage and retrieval of direction relations. Direction relations deal with order in space, as for instance, left, above, north, southeast etc. In this paper we define direction relations between points and extend the definitions to relations between objects. Then we present our tests regarding the retrieval of direction relations between objects using R-trees and discuss the representational properties of MBR approximations with respect to directions in 2D space.
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. A Data Warehouse #DW# can be abstractly seen as a set of materialized views de#ned over relations that are stored in distributed heterogeneous databases. The selection of views for materialization in a DWisthus an important decision... more
. A Data Warehouse #DW# can be abstractly seen as a set of materialized views de#ned over relations that are stored in distributed heterogeneous databases. The selection of views for materialization in a DWisthus an important decision problem. The objective is the minimization of the combination of the query evaluation and view maintenance costs. In this paper we expand on our previous work by proposing new heuristic algorithms for the DW design problem. These algorithms are described in terms of a state space search problem, and are guaranteed to deliver an optimal solution by expanding only a small fraction of the states produced by the #original# exhaustive algorithm. 1 Introduction A Data Warehouse #DW# can be seen as a set of materialized views de#ned over distributed heterogeneous databases. All the queries posed to the DW are evaluated locally using exclusively the data that are stored in the views. The materialized views have also to be refreshed when changes occur to the da...
In a digital era, Surveying Engineering, as other disciplines, faces a paradigm shift. This abolition of complex techniques on one hand, as well as the significant involvement in the geospatial data handling on the other, has lead to a... more
In a digital era, Surveying Engineering, as other disciplines, faces a paradigm shift. This abolition of complex techniques on one hand, as well as the significant involvement in the geospatial data handling on the other, has lead to a redefined field of activities known as Geo-Information or Geomatics. This has been proposed as the new paradigm for Surveying Engineering. Such changes affect undergraduate and postgraduate studies world-wide. Based on these realisations, a new Postgraduate Programme has been established at the National Technical University of Athens (NTUA). Although administered by the Faculty of Rural and Surveying Engineering, the Programme has an interdisciplinary character. After the first two years of operation, a number of very useful experiences can be drawn about the new role and objectives of such programmes. These realisations are not only of local importance but also exhibit of a wider interest. The issues reported in this paper include curriculum developm...
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ABSTRACT We consider a social networking service where numerous subscribers consent to disclose their current geographic location to a central server, but with a varying degree of uncertainty in order to protect their privacy. We aim to... more
ABSTRACT We consider a social networking service where numerous subscribers consent to disclose their current geographic location to a central server, but with a varying degree of uncertainty in order to protect their privacy. We aim to effectively provide instant response to multiple user requests, each focusing at continuously monitoring possible presence of their friends or followers in a time-varying region of interest. Every continuous range query must also specify a cutoff threshold for filtering out results with small appearance likelihood; for instance, a user may wish to identify her friends currently located somewhere in the city center with a probability no less than 75%. Assuming a continuous uncertainty model for streaming positional updates, we develop novel pruning heuristics based on spatial and probabilistic properties of the data so as to avoid examination of non-qualifying candidates. Approximate answers are reported with confidence margins, as a means of providing quality guarantees and suppressing useless messages. We complement our analysis with a comprehensive experimental study, which indicates that the proposed technique offers almost real-time notification with tolerable error for diverse query workloads under fluctuating uncertainty conditions.
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A panel session concerning the use of knowledge-based techniques is reported. The discussion covers the following topics: knowledge-based machine vision architectures; experiences from a knowledge-based integrated-circuit testing system;... more
A panel session concerning the use of knowledge-based techniques is reported. The discussion covers the following topics: knowledge-based machine vision architectures; experiences from a knowledge-based integrated-circuit testing system; artificial-intelligence (AI) tools for requirements capture and analysis; an approach to knowledge modeling in expert systems design; and a homogeneous approach to bringing AI and database management systems together
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A methodology recently proposed to improve processing of star queries on data warehouses is the clustering and indexing of fact tables using their multidimensional hierarchies [DRSN98, MRB99, KS01]. Due to this improved organization... more
A methodology recently proposed to improve processing of star queries on data warehouses is the clustering and indexing of fact tables using their multidimensional hierarchies [DRSN98, MRB99, KS01]. Due to this improved organization schema, processing of aggregation star queries changes dramatically creating new optimization opportunities. An important optimization technique is the so-called pre-grouping transformation. Although this transformation is expected to improve the query-processing plan in most cases, there are several cases where it is not beneficial.
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One of the recently proposed techniques for the efficient evaluation of OLAP aggregate queries is the usage of clustering access methods. These methods store the fact table of a data warehouse clustered according to the dimension... more
One of the recently proposed techniques for the efficient evaluation of OLAP aggregate queries is the usage of clustering access methods. These methods store the fact table of a data warehouse clustered according to the dimension hierarchies using special attributes called hierarchical surrogate keys. In the presence of these access methods new processing and optimization techniques have been recently proposed. One important such optimization technique, called Hierarchical Pre-Grouping, uses the hierarchical surrogate keys in order to aggregate the fact table tuples as early as possible and to avoid redundant joins. In this paper, we study the Pre-Grouping trans- formation, attempting to generalize its applica- bility and identify its relationship to other similar transformations. Our results include a general algebraic definition of the Pre-Grouping transfor- mation along with the formal definition of suffi- cient conditions for applying the transformation. Using a provided theorem...
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Personalization on the Web is an issue that has gained a lot of interest lately. Web sites have already started providing services such as preferences for the interface, the lay- out and the functionality of the applications.... more
Personalization on the Web is an issue that has gained a lot of interest lately. Web sites have already started providing services such as preferences for the interface, the lay- out and the functionality of the applications. Personalization services have also been intro- duced in Web search and metasearch engines, i.e. tools that retrieve Web pages relevant to keywords given
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Hierarchical structures are a way to organize and enrich semantically the available information on the Web. Popular examples of such structures are the product catalogs of e-market stores, which provide data (i.e. products) organized in... more
Hierarchical structures are a way to organize and enrich semantically the available information on the Web. Popular examples of such structures are the product catalogs of e-market stores, which provide data (i.e. products) organized in thematic hierarchies on a category/subcategory basis. Users can navigate these hierarchies to identify data of their preference. Such hierarchies group data under certain properties. Navigational
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Hierarchical structures is a popular means of organizing data in Digital libraries for browsing and querying. Examples of such struc- tures are the catalogs which provide data organized in thematic hier- archies on a category/subcategory... more
Hierarchical structures is a popular means of organizing data in Digital libraries for browsing and querying. Examples of such struc- tures are the catalogs which provide data organized in thematic hier- archies on a category/subcategory basis. Such hierarchies group data under certain properties. Navigational paths are the knowledge artifacts to represent such groups. We consider paths in hierarchies as patterns
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The two fundamental challenges in Virtual Reality over theWWW are (i) to facilitate the process of creating convincing,non-trivial, highly interactive and maintainable content and (ii)to efficiently deliver this content to the client with... more
The two fundamental challenges in Virtual Reality over theWWW are (i) to facilitate the process of creating convincing,non-trivial, highly interactive and maintainable content and (ii)to efficiently deliver this content to the client with QoSguarantees. This paper focuses on the first challenge: we havecreated a data model for spatio-temporal compositions thatallows us to define relative placement of objects in a virtualworld