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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

GIS Data Models III

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GIS Data Models III

GEOG 370

Instructor: Christine Erlien

- Represent spatial locations explicitly
- Relationships between entities implicit
- Space between geographic entities not stored

- Multiple data models
- Examination of relationships
- Between variables in 1 map
- Among variables in multiple maps

- Examination of relationships
- Data models
- Spaghetti models
- Topological models
- Vector chain codes

- 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

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

- 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

- Line segments have beginning & ending

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

- Multiple models
- GBF/DIME (geographic base file/dual independent map encoding)
- TIGER (topologically integrated geographic encoding and reference system)
- POLYVRT (POLYgon conVERTer)

- 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)

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)

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)

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

- 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

- Each line segment

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)

- Hybrid system
- Links graphic data structures with DBMS
- Efficiently manage both graphics & attribute data
- Allows raster & vector data types

- Links graphic data structures with DBMS

From Fundamentals of Geographic Information Systems, Demers (2005)

- 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

- 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