Transcript GEOG370_Ch4p3

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
very short
Line
longer
Area
collection of line segments
– Each entity is a single record, coded as variablelength 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
• 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
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
– Both street addresses &
UTM coordinates defined
for each link
– 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
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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
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– Variable types & operations particular to
that class
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Example: ArcGIS’ geodatabase
– Shift from previous hybrid orientation