Attribute Data

CS 128/ES 228 - Lecture 6a3Database ReviewDatabases consist of tablesEach table holds records Records are in rows Each record consists of fields, i.e. individual data itemsFields are in columns

Fields that have unique values within a table are called keysCampusIDNameTypeFloorsFootprint6MurphyAcademic220019HopkinsSupport294612MaintenanceSupport1184815HickeySupport2236717Shay-LoughlenDorm312983CS 128/ES 228 - Lecture 6a2Why 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 locations2CS 128/ES 228 - Lecture 6a4Sequential DatabasesOnce upon a time, computer scientists tried to put all the data for an application in one table

It didnt work4CS 128/ES 228 - Lecture 6a5Flaws of Sequential DatabasesNot all data is homogeneous

Monolithic structure leads to contention problems

Separation of data into sub databases leads to duplication (and inconsistencies)5CS 128/ES 228 - Lecture 6a6Relational databasesUse many (related) tables of data, with minimal duplication

Tables are linked through common values in particular fields

Queries permit rich data mining

6CS 128/ES 228 - Lecture 6a7What Color Hair Does Student 234 Have?

MaryDawnPetePat117Mary3.8201Dave2.5234Dawn3.1235Pete3.2382Ted2.6378Jane3.0444Ann4.0This is an example of a JOIN operation7CS 128/ES 228 - Lecture 6a8Relational Databases in GISOne table traditionally holds geographic information Other tables hold data about other attributesTables are linked through Object Ids(Object Ids should be independent of the software)8CS 128/ES 228 - Lecture 6a9Relational Databases in GISConsider a Buildings layerGIS data includes location (including coordinate system), symbology, internal bookkeeping

External data includes name of building, date built, purpose, etc.9CS 128/ES 228 - Lecture 6a10

Sometimes they are mixed10CS 128/ES 228 - Lecture 6a11QueriesInformation is gained by linking tables through joins

Queries can involve computed quantities, etc.

11CS 128/ES 228 - Lecture 6a12Storing TopologyShapefiles contain no topological information

But topology is important

SOLUTION

Store the topology elsewhere12CS 128/ES 228 - Lecture 6a13PracticalityGIS must maintain topology (to be useful)

GIS cant store topology (in shapefiles)

SO, GIS must compute topology13CS 128/ES 228 - Lecture 6a14Data Structure for Topology19876453210ABCDZone 1Zone 2Bound.AB4AC5BC10CD8Select Boundary Where Zone1=AAns: 4,514CS 128/ES 228 - Lecture 6a15Topological OperationsSplit one region into two

ABCNameColorAreaAGreen275NameColorAreaBGreen190CGreen8515CS 128/ES 228 - Lecture 6a16Topological OperationsMerge two regions into one

ABCNameColorAreaAGreen190BRed85NameColorAreaC???27516CS 128/ES 228 - Lecture 6a17Dealing with topological operationsUnion(whose data dominates if there is a conflict?)Intersect (whose data do we keep?)SnapsClipping (cropping) a raster OR a vector17Issues with large GISsCurrencyHow do keep the data up to date?CS 128/ES 228 - Lecture 6a18

18Where does currency arise in GISs?Real-time weather (or other environmental) dataTracking vehiclesTracking prisoners be careful where you put the tracking collar!CS 128/ES 228 - Lecture 6a19Many Places19Issues with large GISsConcurrencyHow do we prevent conflicts?

One way is to permit read onlyWorks for systems like FRIARDoes not work when transactions must be madeCS 128/ES 228 - Lecture 6a2020A Concurrency SkitScene: One seat (8B) is left on Fridays 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 6a2121A Concurrency Skit(continued)Time 3: Teds agent books his seat.Time 4: Daves agent books his seat.Scene 2: Friday afternoon at the airport...(action to be ad-libbed)CS 128/ES 228 - Lecture 6a2222Where does concurrency arise in GISs?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 6a2323Issues with large GISsSecurityOf proprietary data

Legal/ethical concernsMassachusetts mileage taxRental car trackingTraffic reports (what if theyre wrong?)CS 128/ES 228 - Lecture 6a2424CS 128/ES 228 - Lecture 6a25What distinguishes (some) GISsThe types of these operations that they can perform.

Both Database and Topological25