GIS Data Models III
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Transcript of GIS Data Models III
GIS Data Models III
GEOG 370Instructor: 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 spaghettiPoint 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
• 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
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
Area
Line
Point
Coordinates
From Fundamentals of Geographic Information Systems, Demers (2005)
Topological Data Models: POLYVRT
From Fundamentals of Geographic Information Systems, Demers (2005)
Entities stored separately but linked to one another through pointers
Chains: Collections of line segments with directional information (from-to, left/right)
ShapefileComprised 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 storedCreated on-the-flyShapefile in-between spaghetti & topological data structures
ArcGIS/Arcview’s shapefile
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
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 Model to Represent Surfaces: TIN
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