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What is Where?

What is Where?. Getting Started With Geographic Information Systems Chapter 5. A GIS can answer the question: What is where?. WHAT: Characteristics of attributes or features. WHERE: In geographic space. Attribute Data Flat File Relations. Map Data Point File Line File Area File

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What is Where?

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  1. What is Where? Getting Started With Geographic Information Systems Chapter 5

  2. A GIS can answer the question: What is where? • WHAT: Characteristics of attributes or features. • WHERE: In geographic space.

  3. Attribute Data Flat File Relations Map Data Point File Line File Area File Topology A GIS links attribute and spatial data

  4. What is a Data Model? • A logical construct for the storage and retrieval of information. • GIS map data structures are map data models. • Attribute data models are needed for the DBMS. • The origin of DBMS data models is in computer science.

  5. Historically, databases were structured hierarchically in files... USA Oregon California New York Ventura Santa Barbara Contra Costa

  6. Most current DBMS is by relational databases. • Based on multiple flat files for records, with dissimilar attribute structures, connected by a common key attribute.

  7. Relational Data Bases File Patient Record Key Check-in Check Out Room No. 42 2/1/96 2/4/96 N763 78 2/3/96 2/4/96 N712 Purchase Record File Item Date Price Customer Key Skate Board 2/1/96 49.95 John Smith 42 Baseball Bat 2/1/96 17.99 James Brown 978 File Accident Report Date Injury Name Key Location 2/1/96 Broken Leg John Smith 42 75 Elm Street 2/2/96 Concussion Sylvia Jones 654 12 State Street 2/2/96 Cut on Ear Robert Doe 123 2323 Broad Street

  8. A DBMS contains: • Data definition language • Data dictionary • Data-entry module • Data update module • Query language • Report generator

  9. Data definition language • Allows the user to establish the structure and names of records and attributes.

  10. Data dictionary • Directory of meta-data (information about the data: what data of what types (numerical, text) are available for what areas and time periods?)

  11. Data-entry module • Needs to “catch” data-entry mistakes as they happen, based on the dictionary (“What kind of data belongs in this entry?”)

  12. Data-update module • Allows corrections and additions or insertions. • Allows new records? • Allows new attributes? • Allows changes in entered values? • Tracks changed values?

  13. Query language • Specific operators that allow the user to retrieve information from the data. • “How many records with certain attributes?” • “Create a new attribute by combining attributes.”

  14. DBMS queries via the query language • sorting • renumbering • subsetting • searching

  15. Command line attribute query compute in states population_density = population / area <50 records in result> restrict in states where population_density > 1000 <20 records selected in result> recode population_density = 3 <20 values recoded in result> join result with states replace <20 records changed in state> restrict in states where population_density > 100 <12 records in result> recode population_density = 2 <12 values changed in result> join result with states replace <12 records changed> compute in states where population_density != 3 or 2 population_density = 1 < 18 records changed>

  16. Retrieval • The ability of the DBMS or GIS to get back on demand data that were previously stored. • Geographic search is the secret to GIS data retrieval. • Many forms of data organization are incapable of geographic search. • GIS systems have embedded DBMSs, or link to a commercial DBMS.

  17. Retrieval Operations • Searches by attribute: find and browse. • Data reorganization: select, renumber, and sort. • Compute allows the creation of new attributes based on calculated values.

  18. Spatial Retrieval Operations • Attribute queries are not very useful for geographic search. • In a map database the records are features. • The spatial equivalent of a find is locate, the GIS highlights the result. • Spatial equivalents of the DBMS queries result in locating sets of features or building new GIS layers.

  19. Spatial Search • Buffering is a spatial retrieval around points, lines, or areas based on distance. • Overlay is a spatial retrieval operation that is equivalent to an attribute join.

  20. Recode OR

  21. Data overlay

  22. Overlay

  23. Types of overlay operations • And • Or • Max • Min • Exhaustive set

  24. Buffer + 1

  25. Complex Retrieval: Map Algebra • Combinations of spatial and attribute queries can build some complex and powerful GIS operations, such as weighting.

  26. Report generator • Allows user to control the format for printing the results of queries. • In a GIS, this parallels the map-creating and editing capabilities of the system.

  27. Coming next… • WHY IS IT THERE?

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