CS 128/ES Lecture 6a1 Attribute Data CampusIDNameTypeFloorsFootprint 6MurphyAcademic HopkinsSupport MaintenanceSupport HickeySupport Shay-LoughlenDorm31298

CS 128/ES Lecture 6a2 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 Lecture 6a3 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” CampusIDNameTypeFloorsFootprint 6MurphyAcademic HopkinsSupport MaintenanceSupport HickeySupport Shay-LoughlenDorm31298

CS 128/ES Lecture 6a4 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 Lecture 6a5 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 Lecture 6a6 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 Lecture 6a7 What Color Hair Does Student 234 Have? Mary Dawn Pete Pat 117Mary Dave Dawn Pete Ted Jane Ann4.0 This is an example of a JOIN operation

CS 128/ES Lecture 6a8 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 Lecture 6a9 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 Lecture 6a10 Sometimes they are mixed

CS 128/ES Lecture 6a11 Queries Information is gained by linking tables through “joins” Queries can involve computed quantities, etc.

CS 128/ES Lecture 6a12 Storing Topology Shapefiles contain no topological information But topology is important SOLUTION Store the topology elsewhere

CS 128/ES Lecture 6a13 Practicality  GIS must maintain topology (to be useful)  GIS can’t store topology (in shapefiles)  SO, GIS must compute topology

CS 128/ES Lecture 6a14 Data Structure for Topology A B C D Zone 1 Zone 2 Bou nd. AB4 AC5 BC10 CD8 Select Boundary Where Zone1=A Ans: 4,5

CS 128/ES Lecture 6a15 Topological Operations Split one region into two A B C NameColorArea AGreen275 NameColorArea BGreen190 CGreen85

CS 128/ES Lecture 6a16 Topological Operations Merge two regions into one A B C NameColorArea AGreen190 BRed85 NameColorArea C???275

CS 128/ES Lecture 6a17 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

Issues with large GIS’s Currency How do keep the data up to date? CS 128/ES Lecture 6a18

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! … CS 128/ES Lecture 6a19 Many Places

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 Lecture 6a20

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 Lecture 6a21

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 Lecture 6a22

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 Lecture 6a23

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 Lecture 6a24

CS 128/ES Lecture 6a25 What distinguishes (some) GIS’s The types of these operations that they can perform. Both Database and Topological