GIS Data Models III

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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

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
• Examination of relationships
• Between variables in 1 map
• Among variables in multiple maps
• Data models
• Spaghetti models
• Topological models
• Vector chain codes
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

Point Line Area

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
Vector Data Model: Spaghetti

From Fundamentals of Geographic Information Systems, Demers (2005)

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

Vector Data Model: Topological
• Topology: Spatial relationships between points, lines & polygons
• Topological models record adjacency information into data structure
• Line segments have beginning & ending
• Node: Point that links two or more lines
• Identifies that point as the beginning or ending of line
• Left & right polygons stored explicitly
Vector Data Model: Topological

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

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)
Topological Data Models: GBF/DIME
• Created by U.S. Census Bureau
• Topology + direction
• Left/right
• From/to

From Fundamentals of Geographic Information Systems, Demers (2005)

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)

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)

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

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
Vector Model to Represent Surfaces: TIN

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

Vector Model to Represent Surfaces: TIN

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

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)

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
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