Geographic Information Systems

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Geographic Information Systems

• GIS Data Models

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1. Components of Geographic Data

• Spatial locations
• Attributes
• Topology
• Time

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Components Spatial Locations

• Specified with reference to a common coordinate
  system
• Spatial features can be represented as
•         points      lines     polygons     vo
  lumes
• grids

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Components - Attributes

• Nominal
• - qualitative, e.g., grasses, trees,
  buildings,      
• Ordinal
• - rankble quality, e.g., high, medium, low   
• Interval
• - quantitative but not ratio, e.g., 400F, 800F
•     
• Ratio
• - absolute quantities, e.g., p, e, -96.345,

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Components - Topology

• Topology
• Spatial relationship between geographic
  features
•   Adjacency
• Containment
• Connectivity
• etc.
• Time
• Presently treated as an attribute

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2. Data Models

• Real world entities and spatial entities

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Data Models Objects and Fields

• Object and Field View
• Objects
• - Are discrete or have identifiable boundaries
• - Have attributes
• Fields
• - Are continuously
• - Attributes vary across the field

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Data Models

• Vector data model
• Raster data model

courtesy Mary Ruvane, http//ils.unc.edu/
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Data Models - (1) Vector

• Every position has a pair of coordinates.
• Lines and polygons are constructed by connecting
  a series of points.
• Points, lines, and polygons are used to
  represent geographic features.

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Epidemiological Studies- Disease Tracking
Incidence of Viruses
ESRI, GE SmallWorld
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Data Models - (2) Raster

• A matrix consists of regular grid cells
• Positions are defined by column and row numbers
• Each cell has a single value

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Data Models - (2) Raster

• Data consist of mostly attributes
• A header file stores (1) Number of rows and
  columns, (2) Cell size, and (3) Coordinates of
  the origin
• Coordinates of a location can be converted on the
  fly

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Data Models - (2) Raster

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2 Data Model(3) Advantages and Disadvantages

• Raster
• Vector

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Ad and Disad - Raster

• Advantages
• - It is a simple data model     - Overlay
  operation can be easily implemented   - High
  spatial variation is efficiently represented
• Disadvantages
• - It is less compact - Topological
  relationships are difficult to represent      -
  The output is less aesthetically pleasing

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Ad and Disad - Vector

• Advantages
• - It is a compact data model      - It is
  efficient in coding topology   
• - The output closely approximate hand-drawn
  maps
•  Disadvantages
• - It is a complex data model      - Overlay
  operations are difficult to implement      - The
  representation of high variation is inefficient

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Readings

• Chapter 3

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Geographic Information Systems

• GIS Data Structures

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GIS Data Structures

• World views
• - object
• - field
• Data models
• - vector
• - raster
• Data structures  
• -  non-topological and topological
• -  run-length and quadtrees

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GIS Data Structures

• Raster data structures
• - Run length     - Quadtrees
• Vector data structures
• - Non-topological structure
• - Topological structure      

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3 Raster Data Structures(1) Run Length

• Data are recorded in the order of
• Attribute1, number of cells in the run
• Attribute2, number of cells in the run  
•  

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3 Raster (1) Run Length

•  

• 11,4 3,4
• 11,4 3,4
• 11,4 3,4
• 11,4 3,4
• 11,3 3,1 5,4

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3 Raster Data Structures(2) Quadtrees

• Subdividing a region into quadrants until each
  quadrant contains only one class - variable
  resolution
•  

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3 (2) Quadtrees
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3 (2) Quadtrees
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4 Vector Data Structures(1) Non-topological
Structure

• There is no topology, and shared boundaries are
  recorded twice
•  

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4 Vector (1) Non-Topological Structure

• Storing coordinates of
• two polygons and
• Polygon
• x1, y1 x2,y2 x3,y3 . x1,y1.
• Polygon
• x1, y1 x2,y2 x3,y3 . x1,y1.

A
(x1, y1)
(x1, y1)
1
2
1
2
B
(x2, y2)
(x2, y2)
1
2
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4 Vector (2) Topological Structure

• No line segment is duplicated
• Line segments and nodes can be referenced to more
  than one polygons
• All polygons have unique identifiers
• Island and hole polygons can be uniquely
  represented

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4 Vector (2) Topological Structure

• Storing coordinates of two polygons and
• Line 1
• x1, y1 x2,y2 x3, y3 x4, y4 x5,y5
• Line 2
• x1, y1 x5,y5
• Line 3
• x5, y5 x6,y6 x7, y7 x1, y1
• Polygon Line 1, Line 2
• Polygon Line 2, Line 3

1
2
(x1, y1)
(x1, y1)
Line 1
1
Line 2
Line 3
2
(x5, y5)
(x5, y5)
1
2
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4 Vector (2) Topological Structure

• Polygon a list of lines
• Line a set of coordinates
• Point a line
• with no length

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Topological- the Arc-Node Model

• Arc - a line starting and ending at a node
• Node - intersection point where two or more arcs
  meet
• or it is a dead end
• Polygon - a closed chain of arcs

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Arc-Node Model

•   Polygon topology table Node topology
  table Arc topology table
• Arc coordinate data table

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•  

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Readings

• Chapter 3