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