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”

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 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 6a8 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 6a9 Sometimes they are mixed

CS 128/ES Lecture 6a10 Queries Information is gained by linking tables through “joins” Queries can involve computed quantities, etc.

CS 128/ES Lecture 6a11 Storing Topology Shapefiles contain no topological information But topology is important SOLUTION Store the topology elsewhere

CS 128/ES Lecture 6a12 Data Structure for Topology A B C D Zone 1 Zone 2 Bou nd. AB4 AC5 BC10 CD8

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 Topological Operations Merge two regions into one A B C

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

CS 128/ES Lecture 6a16 Topological Operations Clip an image

CS 128/ES Lecture 6a17 Topological Operations Erase an image

CS 128/ES Lecture 6a18 Other operations Union Intersect “Snaps”

CS 128/ES Lecture 6a19 How this is used Find nearest neighbor Find largest “empty” region

CS 128/ES Lecture 6a20 What distinguishes (some) GIS’s The types of these operations that they can perform. Both Database and Topological