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CampusID. Name. Type. Floors. Footprint. 6. Murphy. Academic. 2. 2001. 9. Hopkins. Support. 2. 946. 12. Maintenance. Support. 1. 1848. 15. Hickey. Support. 2. 2367. 17. Shay-Loughlen. Dorm. 3. 1298. Attribute Data. Why Databases?.

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Attribute Data

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  1. CampusID Name Type Floors Footprint 6 Murphy Academic 2 2001 9 Hopkins Support 2 946 12 Maintenance Support 1 1848 15 Hickey Support 2 2367 17 Shay-Loughlen Dorm 3 1298 Attribute Data CS 128/ES 228 - Lecture 6a

  2. Why Databases? • One of the advantages of vector-based data is that each datum corresponds to a (portion) of a “real” object. • BUT… Objects are more than geographic locations CS 128/ES 228 - Lecture 6a

  3. CampusID Name Type Floors Footprint 6 Murphy Academic 2 2001 9 Hopkins Support 2 946 12 Maintenance Support 1 1848 15 Hickey Support 2 2367 17 Shay-Loughlen Dorm 3 1298 Database “Review” • Databases consist of tables • Each table holds records • Records are in rows • Each record consists of fields, i.e. individual data items • Fields are in columns • Fields that have unique values within a table are called “keys” CS 128/ES 228 - Lecture 6a

  4. Sequential Databases Once upon a time, computer scientists tried to put all the data for an application in one table It didn’t work CS 128/ES 228 - Lecture 6a

  5. Flaws of Sequential Databases • Not all data is homogeneous • Monolithic structure leads to contention problems • Separation of data into “sub” databases leads to duplication (and inconsistencies) CS 128/ES 228 - Lecture 6a

  6. Relational databases • Use many (related) tables of data, with minimal duplication • Tables are “linked” through common values in particular fields • “Queries” permit rich “data mining” CS 128/ES 228 - Lecture 6a

  7. What Color Hair Does Student 234 Have? This is an example of a JOIN operation CS 128/ES 228 - Lecture 6a

  8. Relational Databases in GIS • One table traditionally holds geographic information • Other tables hold data about other attributes • Tables are linked through “Object Ids” • (Object Ids should be independent of the software) CS 128/ES 228 - Lecture 6a

  9. Relational Databases in GIS • Consider a “Buildings” layer • “GIS” data includes location (including coordinate system), symbology, internal bookkeeping • “External” data includes name of building, date built, purpose, etc. CS 128/ES 228 - Lecture 6a

  10. Sometimes they are mixed CS 128/ES 228 - Lecture 6a

  11. Queries • Information is gained by linking tables through “joins” • Queries can involve computed quantities, etc. CS 128/ES 228 - Lecture 6a

  12. Storing Topology • Shapefiles contain no topological information • But topology is important SOLUTION • Store the topology elsewhere CS 128/ES 228 - Lecture 6a

  13. Practicality • GIS must maintain topology (to be useful) • GIS can’t store topology (in shapefiles) • SO, GIS must compute topology CS 128/ES 228 - Lecture 6a

  14. 2 1 B 4 A 10 3 5 9 C 8 D 6 7 Data Structure for Topology Select Boundary Where Zone1=A Ans: 4,5 CS 128/ES 228 - Lecture 6a

  15. B A C Topological Operations • Split one region into two CS 128/ES 228 - Lecture 6a

  16. C A B Topological Operations • Merge two regions into one CS 128/ES 228 - Lecture 6a

  17. Dealing with topological operations • Union (whose data dominates if there is a conflict?) • Intersect (whose data do we keep?) • “Snaps” • Clipping (cropping) a raster OR a vector CS 128/ES 228 - Lecture 6a

  18. Issues with large GIS’s • Currency • How do keep the data up to date? CS 128/ES 228 - Lecture 6a

  19. Where does currency arise in GIS’s? • Real-time weather (or other environmental) data • Tracking vehicles • Tracking prisoners – be careful where you put the tracking collar! • … • Many Places CS 128/ES 228 - Lecture 6a

  20. Issues with large GIS’s • Concurrency • How do we prevent conflicts? • One way is to permit “read only” • Works for systems like FRIAR • Does not work when “transactions” must be made CS 128/ES 228 - Lecture 6a

  21. A Concurrency Skit • Scene: One seat (8B) is left on Friday’s flight to Cancun. • Time 1: Ted calls up travel agent and asks if any seats are left. Is told yes. • Time 2: Dave calls up different travel agent and asks if any seats are left. Is told yes. CS 128/ES 228 - Lecture 6a

  22. A Concurrency Skit(continued) • Time 3: Ted’s agent books his seat. • Time 4: Dave’s agent books his seat. • Scene 2: Friday afternoon at the airport... (action to be ad-libbed) CS 128/ES 228 - Lecture 6a

  23. Where does concurrency arise in GIS’s? • The are not that common, but some examples include: • Issuing orders to vehicles in the field. • Updating claims on gold mines CS 128/ES 228 - Lecture 6a

  24. Issues with large GIS’s • Security • Of proprietary data • Legal/ethical concerns • Massachusetts mileage tax • Rental car tracking • Traffic reports (what if they’re wrong?) CS 128/ES 228 - Lecture 6a

  25. What distinguishes (some) GIS’s • The types of these operations that they can perform. Both Database and Topological CS 128/ES 228 - Lecture 6a

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