Implementation and testing of variable scale topological data structures

Transcription

1 Implementation and testing of variable scale topological data structures Experiences with the GAP-face tree and GAP-edge forest Master s thesis proposal by B.M. Meijers November 2005 Delft University of Technology Faculty of Civil Engineering and Geosciences Geodetic Engineering Section GIS Technology The Netherlands

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3 Implementation and testing of variable scale topological data structures Experiences with the GAP-face tree and GAP-edge forest Master s thesis proposal Martijn Meijers 29th November General information Name master student: Bernard Marinus (Martijn) Meijers Title thesis project: Implementation and testing of variable scale topological data structures: Experiences with the GAP-face tree and GAP-edge forest Supervising professor: Prof. dr. ir. P.J.M. van Oosterom Supervisor: Drs. C.W. Quak Start of thesis project: 9th November 2005 End of thesis project: 9th June 2006 Planned graduation date: October Problem and its context Spatial Data Infrastructures (SDIs), which enable users to combine geographic data from several sources over the internet and mobile use of geographic data (over wireless broadband networks, like Universal Mobile Telecommunications System, UMTS) require a new approach of generalisation (Stoter et al., 2004). It is nowadays possible to transfer geographical data from a server to a user, who is carrying a mobile device with a small screen. The small screens of these devices do not offer much space to show geographical data, therefore it should be possible for a user to rapidly zoom in and out. As is shown in figure 1, when the user changes the scale (e.g. when a user zooms out), the degree of information shown on the screen should be about the same, otherwise the screen will be cluttered with irrelevant information for that scale and, more important, the amount of data to be transfered will be too large 3

4 (the user suffers from information overload). Assume a user zooms out, like is done in figure 1, important features still should be shown, while less important features could be left out on a smaller scale. Generalization should take place to solve the problem of information overload. (a) Large scale (b) Mid-scale (c) Small scale Figure 1: Delft at different scales. Information density should remain about the same at different scales; Generalization as a solution Three types of generalization can be distinguished: batch, interactive and onthe-fly generalization. Batch generalization is a process where a computer returns an output dataset using algorithms, rules, or constraints based on an input dataset (Lagrange et al., 1993, cited in Van Oosterom (1995)). In interactive generalization the process is steered by a user, who interacts with the computer and provides knowledge to arrive at the output dataset. Interactive and batch generalization offer ways to create geographical datasets at different scales. Often a waterfall-like process is used to arrive at some predefined scale levels. This means that a user is bound to the selected scales and that these methods do not offer a way to use geographical data in a scaleless manner. Another problem with these processes is that a copy of the dataset for the new scale is made, which results in redundant storage of data, in its turn resulting in problems with keeping the data up-to-date. Another approach that can be taken is on-the-fly generalization. This method does not have the problems mentioned above, because it tries to create a temporary generalization, based on one detailed geographic dataset (Van Oosterom, 1995). However, it is not possible to calculate these data structures on-the-fly, because this would be too calculation intensive (and thus too slow). Therefore, some data structures have been proposed, like the GAP-, Reactive- and BSP-tree (Van Oosterom and Schenkelaars, 1995), in which the data can be loaded, so the calculation step does not need to be done each time a user wants a generalization. Up to now these data structures (or variants of these structures) were not fully structured nonredundant for geometry and did not perform in an implementation in a way that they were suitable for use in a production environment (Vermeij, 2003). Based on 4

5 the previous structures a new combination has been proposed by Van Oosterom (2005). A combination of an extended GAP-tree (called the GAP-face tree) and BLG-trees (combined into a forest of trees) structured in a topological way should solve the problems of the earlier attempts. However, this proposed solution has not been verified in theory and it has not been benchmarked yet. The goal of this research therefore will be: To test the GAP-face tree and GAP-edge forest their performance for on-the-fly database map generalization by verification of the theory and implementing the data structures in an object-relational database management system and running several tests on top of the developed data structures. 3 Assignment In this section the approach that will be taken during the research (i.e. the research framework and the research issues) will be described. 3.1 Research framework To reach the objective of the research a framework can be established, which will be of guidance during the research. This framework consists of four parts: the research objects (a), the research perspective (b), interrelation of research results (c) and the objective of the research (d). The objective of this research can be characterized as: the implementation and testing of a new reactive data structure. This objective will be reached by following the path of conducting literature study on the proposed data structures in the light of map generalization theory and giving a theoretical description of implementation in an object-relational database management system. This theoretical background (a) then provides enough points of interests for implementing and testing the data structures (b). Based on the implementation experiment and test results (c) conclusions and recommendations regarding the performance of the data structures can be given (d). In this framework, the ingredients to start the research with (the research objects) are: Theory on map generalization; Theory on GAP-face tree, GAP-edge tree and forest; Theory on winged edge topology; Theory on Douglas Peucker algorithm for line simplification; Literature on features needed for the data structures available within objectrelational database management systems; Choice which database management system to use; 5

6 Literature on earlier implementations of about the same concept (Van Putten, 1997; Van Putten and Van Oosterom, 2000a,b; Vermeij, 2003); Literature on the choice of importance values for creating trees (Van Putten, 1997). These ingredients will deliver points of interest for diagnosing the data structures. The main part of the research will be based on an experiment of implementing the data structures in an object-relational database management system. This experiment will consist of the following parts: Selecting and loading of topological test data; Implementation of the GAP-face tree, GAP-edge tree and forest; Test if it is feasible to remove all geometric redundancy in the data structures; Test the loading speed of developed data structures; Test the visualization speed of developed data structures, by using Google Earth. All the individual research results stemming from the implementation and testing phase can be interrelated, so the objective of the research project can be fulfilled: to give conclusions and recommendations regarding the data structures as proposed in Van Oosterom (2005). At the end of the project, deliverables of the research project include: Master s thesis; A software implementation of the data structures (source code), preferably based on existing software; Test results. 3.2 Research issues The research that will be conducted, can be centered around five central questions. These questions each cover a part of the research framework. When for all the questions an answer has been found, the objective of the research as formulated under section 2 should be met. To start the research the following two questions have to be answered by literature study: What are the good things about having these data structures inside the database for map generalization? 6

7 How can the GAP-face tree and GAP-edge forest be implemented in an objectrelational database management system? Subquestions to these central questions: - Which problems of generalization will be solved by implementation of these data structures? - What tables, functions and indexes are needed for an implementation of these data structures? - Which database management system is suited for implementing these data structures? - In which way should we query the data structures if we want to get a fixed number of objects in a spatial domain, not for a fixed importance level? For the experiments the two main questions to be answered are: What problems occur when implementing the data structures inside the database? How can we assess the performance of the implemented structures? Subquestions to these central questions are: - In which way can we measure the performance of the data structures? - Does transmitting of data stored in the new data structures go faster for a certain spatial domain, than transmitting all the data or data stored in a multi-resolution way (redundantly)? - Which parts of the data structures perform well and; - which parts do not perform well? The last central question for interrelating the research results of the literature study and experiments reads: What do we learn from an implementation of the GAP-face tree and GAP-edge forest data structures in an object-relational database management system? Subquestions to this central question: - Is it easy to implement the data structures inside an object-relational database management system? - Are the data structures suitable for implementation in a production environment, e.g. with many concurrent users? 7

8 4 Activities Activities to be undertaken for the research are as follows: Create research proposal Nov 9, 2005 Nov 21, 2005 Create the research proposal to formulate what will be done during the master s thesis project. Study literature Nov 22, 2005 Dec 23, 2005 Study relevant literature on automated polygon generalization and indexing. During this study the implementation and testing phase will be planned in more detail. This entails also study of database management systems their available features and loading of spatial data into these systems. Implement and test data structures Dec 27, 2005 Mar 10, 2006 Implement and test the GAP-face tree and GAP-edge forest in an objectrelational database management system. The progress and results will be reported to the supervisors. Feedback on implementation and raw material Mar 13, 2006 Apr 4, 2006 Get feedback from supervisors and attendants of colloquium presentation II on the raw theoretic material, the implementation and test results. Refine implementation and testing, write draft thesis Apr 5, 2006 Apr 28, 2006 With the feedback from supervisors and colloquium presentation II, refine the implementation and tests. Write final thesis May 8, 2006 May 19, 2006 Transform the draft version of the thesis into a final version, which can be defended. Prepare presentation May 22, 2006 Jun 9, 2006 Prepare the presentation for the thesis defence. 5 Time schedule 5.1 Time Frame The Master s thesis research has been started on 9th November The thesis project is worth 34 credit points. One point means 28 hours of work. The thesis project should be finished in no less (and preferably no more) than = 952 hours. With a working week of 8 hours per day, this means that for the project an amount of time is available that is equal to 119 working days. Because Martijn will 8

9 be busy with moving to a new home during February to June (and less than 8 hours a day will be available), some more time has been scheduled (i.e. 139 working days). Therefore the research should be ended on 9th June Activities in a table The next table shows all the activities that have to be undertaken: Start Finish Days Task Nov 9, 2005 Nov 21, Create research proposal Nov 22, 2005 Dec 23, Study literature Dec 27, 2005 Mar 10, Implement and test data structures Mar 13, 2006 Apr 4, Feedback on implementation and raw material Apr 5, 2006 Apr 28, Refine implementation and testing, write draft thesis May 8, 2006 May 19, Finalise thesis May 22, 2006 Jun 9, Prepare presentation Total: 139 days scheduled. 9

10 5.1.2 Activities in a Gantt chart The activities that have to be undertaken can be visualised in a so-called Gantt chart as is done in figure 2: (a) November, December (b) January, February (c) March, April (d) May, June Figure 2: Gantt chart of the activities 5.2 Important dates Important dates during the project are: Colloquium presentation I Dec 15, 2005 Colloquium presentation II Around Mar 10, 2006 Final thesis May 19, 2006 Presentation Jun 9,

11 5.3 Holidays During the research project some weeks are reserved for holidays. Start Finish Days Dec 26, 2005 Dec 26, Feb 15, 2006 Feb 24, May 1, 2006 May 5, Thesis The provisional contents and a short description per chapter of the thesis are given. As indication between parentheses, an approximate number of pages and a date is given for when the raw material for a chapter should be approximately available: Summary Summary In English and in Dutch (Both 2 pages; Mar 27). Chapter 1 Introduction Introduction of the problem, research objective, research framework and research questions (5 pages; Dec 5). Chapter 2 On automated polygon generalization and spatial indexing techniques Introduction to and backgrounds of automated polygon generalization and spatial indexing techniques. Also the theory of the GAP-face tree, the GAPedge forest and needed indices will be given (10-20 pages; Jan 9). Chapter 3 Methodological justification In this chapter the methods used in the implementation and testing phase of the research are justified (3-5 pages; Dec 23). Chapter 4 Implementation and testing of the GAP-face tree and GAP-edge forest in an object-relational database management system Experiments regarding the implementation and testing of the GAP-face tree and GAP-edge forest will be described (20-30 pages; Mar 13). Chapter 5 Conclusions and recommendations Conclusions and recommendations that can be drawn from the theory and implementation and testing experiments (5 pages; Mar 27). 7 Agreements 7.1 Supervision Drs. C.W. Quak and Martijn will meet every three or four weeks (or more frequent if needed) for having a meeting of progress. Prof. dr. ir. P.J.M. van Oosterom (or a replacement) will join the meetings at the beginning of the project (planning phase), half way (discussing the implementation and testing phase) and near the end of the project (discussing the draft thesis). 11

12 7.2 Instruments For the research the following items are needed: Scientific literature on map generalization; Scientific literature on spatial indexing; PC, with access to the internet, development tools, source of PostgreSQL & PostGIS; Map data, structured in a topological way (preferably winged-edge structure, e.g. a dataset of the municipality of Amsterdam or a part of the Cadastral dataset of The Netherlands). 12

13 Bibliography Lagrange, J., Ruas, A., and Bender, L. (1993). Survey on generalization. Technical report, IGN. Stoter, J., Kraak, M., and Knippers, R. (2004). Generalisation of framework data: a research agenda. In ICA Workshop on Generalisation and Multiple representation, Leicester. Van Oosterom, P. (1991). The reactive-tree: A storage structure for a seamless, scaleless geographic database. In Proceedings Auto-Carto 10, Baltimore, Maryland. Cartography and Geographic Information Society (CaGIS). Van Oosterom, P. (1995). The GAP-tree, an approach to On-the-Fly Map Generalization of an Area Partitioning, volume GIS and Generalization: Methodology and Practice of GISDATA, chapter 9, pages Taylor & Francis, London. Van Oosterom, P. (2005). Scaleless topological data structures suitable for progressive transfer: the gap-face tree and gap-edge forest. In Proceedings Autocarto 2005, Las Vegas, Nevada. Cartography and Geographic Information Society (CaGIS). Van Oosterom, P. and Schenkelaars, V. (1995). The development of an interactive multi-scale gis. International Journal of Geographical Information Systems, 9(5): Van Putten, J. (1997). Experiences with the gap-tree. Master s thesis, University of Utrecht. Van Putten, J. and Van Oosterom, P. (2000a). Generaliseren van vlakkenpartities (1): Gap-trees, theorie en implementatie. Geodesia, 42(10): In Dutch. Van Putten, J. and Van Oosterom, P. (2000b). Generaliseren van vlakkenpartities (2): Gap-trees, testresultaten en verbeteringen. Geodesia, 42(11): In Dutch. Vermeij, M. (2003). Development of a topological data structure for on-the-fly map generalization. Master s thesis, Delft University of Technology. Verschuren, P. and Doorewaard, H. (1999). Designing a Research Project. Lemma, Utrecht. 13

14 A MSc Form The MSc Form is available as a separate document to this proposal. 14