GIS Data Models II

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GIS Data Models II

• GEOG 370
• Instructor Christine Erlien

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Representing Entities Attributes

• What did we talk about last class?
• Managing files
• Attributes
• Graphics
• This class
• How are the graphics represented in GIS?
• Vector
• Raster

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Representing Entities Attributes

• Relationships between entities
• Can be seen on map
• Computer has to be told about them
• Examples
• Lines ? related to one another at an intersection
• Areas? Yellowstone surrounding counties

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Representing Geographic SpaceMethods Raster

• Raster
• Dividing space into a series of units
• Shape square, rectangle, triangle, hexagon
• Generally uniform in size
• Units connected to represent surface of study
  area
• Do not provide precise locational information
• Why?

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Raster Data Structure
A B C D E
1 1 1 2 3
1 3 6 6 6
1 5 5 4 3
1 2 1 1 1
1 1 1 1 1
1 2 3 4 5
Values 1-6 based on color gradation
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Raster Graphic Data Structures Representing
Entities
From Fundamentals of Geographic Information
Systems, Demers (2005)
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Raster LULC
http//landcover.usgs.gov
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Raster Method Attribute Data

• Including attribute data in raster
• Numbers ? represent attributes
• Numbers assigned to each cell

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Raster Method Attribute Data

• Including attribute data in raster
• Link the grid cells to a DBMS
• gt1 attribute represented by each cell
• More prevalent
• Decreases data storage
• Easily linked to other data managed by DBMS

From Fundamentals of Geographic Information
Systems, Demers (2005)
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Representing Geographic SpaceMethods Vector

• Vector (polygon-based)
• Spatial locations are specific
• How?
• Points Single set of X,Y coordinates
• Lines Connected sequence of coordinates
• Areas Sequences of interconnected lines
• 1st last coordinate pair must be same to close
  polygon
• Attributes stored in a separate file

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Representing Geographic SpaceMethods Vector
From Fundamentals of Geographic Information
Systems, Demers (2005)
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Representing Geographic SpaceMethods Vector

• Representing points, lines, areas
• Points Single set of X,Y coordinates
• Lines Connected sequence of coord.
• Straight line ? 1 segment
• Complex lines ? many segments
• Shorter segments represent shape better, but not
  exact
• Areas Sequences of interconnected lines
• Same issues if complex

Still abstracting space
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Representing Geographic SpaceTopology

• Networks Lines that interact
• Include information about attributes how they
  are connected to other lines
• Node Point that links two or more lines
• Identifies that point as the beginning or ending
  of line
• Example Road networks
• Topology Explicit information about
  connectivity relative spatial relationships

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Data Structures vs. Data Models

• Graphic data structures Computer storage of
  analog graphical data that enables close
  approximation of analog graphic to be
  reconstructed
• Data models
• Allow links to attributes
• Allow interactions of objects in database
• Allow for analytical capabilities
• Multiple maps can be analyzed in combination

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www.awi-bremerhaven.de
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Raster Data Models MAP
From Fundamentals of Geographic Information
Systems, Demers (2005)
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Raster Data Models

• Minimizes maps
• Multiple variables associated with each grid cell
• Allows linkage to programs using vector data
  model

From Fundamentals of Geographic Information
Systems, Demers (2005)
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Raster Data Models Data Compression

• Why?
• Save disk space by reducing information content
• Methods
• Run-length codes
• Raster chain codes
• Block codes
• Quadtrees

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Raster Data Compression ModelsRun-length
Encoding
Reduces data volume on a row-by-row basis by
indicating string lengths for various values
From An Introduction to Geographic Information
Systems, Heywood et al. (2002)
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Raster Data Compression Models

• Run-length codes
• Limited to operating row-by-row
• What about areas?
• Raster chain codes A chain of grid cells is
  created around homogenous polygonal areas

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Raster Data Compression ModelsBlock Encoding
Run-length encoding in 2-D Uses a series of
square blocks to encode data
From An Introduction to Geographic Information
Systems, Heywood et al. (2002)
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Raster Data Compression ModelsRaster Chain Codes
Reduces data by defining the boundary of entity
From An Introduction to Geographic Information
Systems, Heywood et al. (2002)
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Raster Data Compression Models

• Quadtrees Recursively divide an area into
  quadrants until all the quadrants (at all levels)
  are homogeneous

NW
NE
SE
1
2
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Raster Data Compression Models
From An Introduction to Geographic Information
Systems, Heywood et al. (2002)
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Raster Imagery Data Compression

• MrSID Multiresolution Seamless Image Database
• Data compressor
• Can handle very large images
• Allows instantaneous browsing
• Reduces storage space necessary
• http//www.lizardtech.com/