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GIS Data Models III GEOG 370 Instructor: Christine Erlien Representing Geographic Space: Vector Data Structures Represent spatial locations explicitly Relationships between entities implicit Space between geographic entities not stored Vector Data Models Multiple data models

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Gis data models iii l.jpg

GIS Data Models III

GEOG 370

Instructor: Christine Erlien


Representing geographic space vector data structures l.jpg

Representing Geographic Space: Vector Data Structures

  • Represent spatial locations explicitly

  • Relationships between entities implicit

    • Space between geographic entities not stored


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Vector Data Models

  • Multiple data models

    • Examination of relationships

      • Between variables in 1 map

      • Among variables in multiple maps

  • Data models

    • Spaghetti models

    • Topological models

    • Vector chain codes


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Vector Data Model: Spaghetti

  • Simplest data structure

  • One-to-one translation of graphical image

    • Doesn’t record topology  relationships implied rather than encoded

  • Each entity is a single piece of spaghetti

    PointLineArea

    very short longer collection of line segments

    • Each entity is a single record, coded as variable-length strings of (X,Y) coordinate pairs

    • Boundaries shared by two polygons  stored twice


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Vector Data Model: Spaghetti

From Fundamentals of Geographic Information Systems, Demers (2005)


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Vector Data Model: Spaghetti

  • Measurement & analysis difficult

    • All relationships among objects must be calculated independently

  • Relatively efficient for cartographic display

    • CAC

  • Plotting: fast

www.gis.niu.edu/Cart_Lab_03.htm


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Vector Data Model: Topological

  • Topology: Spatial relationships between points, lines & polygons

  • Topological models record adjacency information into data structure

    • Line segments have beginning & ending

      • Link: Line segment

      • Node: Point that links two or more lines

        • Identifies that point as the beginning or ending of line

    • Left & right polygons stored explicitly


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Vector Data Model: Topological

From An Introduction to Geographic Information Systems, Heywood et al. (2002)


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Topological Data Models

  • Multiple models

    • GBF/DIME (geographic base file/dual independent map encoding)

    • TIGER (topologically integrated geographic encoding and reference system)

    • POLYVRT (POLYgon conVERTer)


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Topological Data Models: GBF/DIME

  • Created by U.S. Census Bureau

    • Both street addresses & UTM coordinates defined for each link

    • Topology + direction

      • Left/right

      • From/to

From Fundamentals of Geographic Information Systems, Demers (2005)


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Topological Data Models: TIGER

Point

Area

  • Designed for use with the 1990 U.S. Census

  • Block-level maps

    • Points, lines & areas explicitly addressed

    • Census blocks can be retrieved directly by block number

Coordinates

Line

From Fundamentals of Geographic Information Systems, Demers (2005)


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Topological Data Models: POLYVRT

Entities stored separately but linked to one another through pointers

Chains: Collections of line segments with directional information (from-to, left/right)

From Fundamentals of Geographic Information Systems, Demers (2005)


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ArcGIS/Arcview’s shapefile

Shapefile

Comprised of 3 file types

*.shp contains coordinates

*.shx is an index file

*.dbf is an attribute file in dbase format

Where is topology?

Not explicitly stored

Created on-the-fly

Shapefile  in-between spaghetti & topological data structures


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Compacting Vector Data Models

  • Compact data to reduce storage

  • Freeman-Hoffman chain codes

    • Each line segment

      • Directional vector

      • Length

    • Non-topological

      • Analytically limited  limits usefulness to storage, retrieval, output functions

    • Good for distance & shape calculations, plotting


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Vector Model to Represent Surfaces: TIN

TIN allows us to record topographic data as points in a regular or irregular grid.


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Vector Model to Represent Surfaces: TIN

From Geographic Information Systems & Science, Longley et al. (2005)


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Vector GIS: Hybrid & Integrated Systems

  • Hybrid system

    • Links graphic data structures with DBMS

      • Efficiently manage both graphics & attribute data

      • Allows raster & vector data types

From Fundamentals of Geographic Information Systems, Demers (2005)


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Vector GIS: Hybrid & Integrated Systems

  • Integrated system

    • Entities’ coordinate data stored as relational table

    • Topological data stored as separate table in same database

    • Attributes can be

      • Stored in same tables as graphic entities

      • Stored as separate tables & linked relationally

    • GIS more closely integrated with DBMS than in hybrid system


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Vector GIS: Object-Oriented Database Management Systems

  • Emerging as an alternative to hybrid or integrated models

  • Extends the integrated model by incorporating a spatial query language

  • Objects inherit properties from the class of objects that they belong to

    • Variable types & operations particular to that class

  • Example: ArcGIS’ geodatabase

    • Shift from previous hybrid orientation


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