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8/20/2010




   GIS Data Structures and Models




          GE 517 – Geographic Information Systems
                                  Engr. Ablao




Digital Geographic Data
Numerical representations that d ib real-world f
N       i l             i     h describe l         ld features
and phenomena
Must be encoded in digital form and organized as a
geographic database to be useful to GIS
  This digital database creates our perception of the real
  world



  Geographic Information System                        8/20/2010




                                                                          1
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    Conventional Data vs. Digital
    Geographic Data
 Conventional Data: Paper map
     Static representation
         Represents a general-purpose snapshot of the real world at a
         given time only

 Digital Geographic Data:
     Dynamic representation
         Allows a range of functions for storing processing analyzing
                                          storing, processing, analyzing,
         and visualizing spatial data
         Has tools that allow users to interact with the data to meet their
         objectives

       Geographic Information System                                                         8/20/2010




FIGURE 1. Bringing real-world data into a GIS or a map requires
several abstractions and/or conversions of the original data. (Freely
adapted from Bernhardsen, 1992).



  Physical          Real world                                                               Map /
               →                       →   Data model         →       Database        →
    reality            model                                                                  Report

Actual             Entity                  Object                 Object                  Symbol, line,
phenomenon                                                                                   text


• Properties       • Type                  •   Type               •    Type
• Connection       • Attributes            •   Attributes         •    Attributes
  s                • Relationship          •   Relationship       •    Relationship
                                           •   Geometry           •    Geometry
                                           •   Quality            •    Quality




       Geographic Information System                                                         8/20/2010




                                                                                                                 2
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                                     Figure 2
 Geographic Information System                               8/20/2010




     What is a Data Model?
The h
Th heart of any GIS
           f
A way of representing data

A set of constructs for describing and representing
selected aspects of the real world in a computer
                           l ld         computer.

                                 Longley, Goodchild, Maguire & Rhind

 Geographic Information System                               8/20/2010




                                                                                3
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Geographic Data Models
                                   REAL WORLD




   Field-based model                            Object-based model




   Raster data model                            Vector data model

                                    Figure 3
   Geographic Information System                                8/20/2010




Raster and Vector Data Models




   Geographic Information System                                8/20/2010
                                    Figure 4




                                                                                   4
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Raster Data Model
 Treats geographic space as populated by one or more spatial
 phenomena,
 phenomena which vary continuously over space and having
 no obvious boundaries.
 Best used to represent geographic features that are continuous
 over a large area (e.g. soil type, vegetation, etc.)
 Uses an array of rectangular cells/pixels/grids to represent
 real-world objects.
 real world objects
 Each cell is defined by a coordinate location and an attribute
 that identifies the feature.
 Cell’s linear dimensions defines the spatial resolution
    Geographic Information System                      8/20/2010




Raster Data Model




                                    Figure 5
    Geographic Information System                      8/20/2010




                                                                          5
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Raster Data Model




                                         Figure 6
   Geographic Information System         8/20/2010




Raster Data Model




                                   Figure 7
   Geographic Information System         8/20/2010




                                                            6
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    A point is represented as a
    single cell.




    well




           tree




                                                     rock



                                                   Figure 8
        Geographic Information System                       8/20/2010




A line is represented as a string of cells.




                                                                    river
road




canal




                                        Figure 9
        Geographic Information System                       8/20/2010




                                                                                   7
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An area is a contiguous block of cells.


                                                        grassland




   barn


                                                          forest




                                      Figure 10
      Geographic Information System                 8/20/2010




Raster Data Model
  The cell size determines the resolution of the data.
  The grid cell has an extent.
  When we map features onto the grid, there is
  sometimes an imperfect fit.
  Each grid cell can usually be “owned” by one feature,
  that is, the one whose attribute it holds (mixed pixels).



      Geographic Information System                 8/20/2010




                                                                           8
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                                Figure 11
Geographic Information System               8/20/2010




                            Figure 12



Geographic Information System               8/20/2010




                                                               9
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Raster Data Compression
 A single raster file usually contains several million grid
 cells l d to a very l
   ll leading            large d f l
                               data file

  Therefore, data compression is very important in the
 digital representation of raster data!




    Geographic Information System                   8/20/2010




Raster Data Compression
 Based on the realization that cells representing the
 same ffeature type have identical values and that they
                    h     d      l l        d h h
 tend to be spatially clumped
 Examples of raster data compression techniques are:
  Run length encoding
  Wavelet compression




    Geographic Information System                   8/20/2010




                                                                      10
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Raster Data Compression
Run length encoding
       g          g
  Adjacent cells along a row having the same value are
  treated as a group known as a ‘run’
  instead of storing the similar cell values individually,
  the value is just stored once together with the
  number of cells that comprise the run
       b f ll th t             i th



   Geographic Information System                    8/20/2010




Run length encoding




                                   Figure 13

   Geographic Information System                    8/20/2010




                                                                      11
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Raster Data Compression
Wavelet Compression
           p
     More efficient compression
     Can achieve a ratio between 15:1 to 20:1 for 8-bit
     gray scale images and between 30:1 to 40:1 for 24-
     bit color images




      Geographic Information System                                    8/20/2010




Raster File Formats
BMP (bitmaps) – used by graphics in Microsoft Windows applications; no
       p
  compression
DIB (Device Independent Bitmaps)
GIF (Graphical Interchange Format) – widely used for images to be used on the
  World Wide Web
TIFF (Tagged Image File Format) – non-proprietary, system-independent
JPEG (Joint Photographic Experts Group) – primarily for storage of photographic
  images and for the World Wide Web
GeoTIFF – extension of the TIFF format that contains georeferencing information
PNG (Portable Network Graphics) – patent-free; intended to replace the GIF format
PCX – supported by many image scanners
MrSID (Multi-resolution Seamless Image Database)
GRID – proprietary format used by ESRI in ArcInfo and ArcView GIS

      Geographic Information System                                    8/20/2010




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Quadtree Data Model
  A hierarchical field-based model that uses grid cells
  of variable sizes
    f i bl i
  Uses finer subdivisions in areas where finer detail
  occurs
  Geographic space is divided by the process of
  recursive decomposition
  More difficult compared to the simple raster model



    Geographic Information System                  8/20/2010




Quadtree Data Model




                                              Figure 14
    Geographic Information System                  8/20/2010




                                                                     13
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Digital Elevation Model

  A raster-based
  representation of
  surface




                       Figure 15
    Geographic Information System
                                      Southwest Corner of the 8/20/2010
                                    Morrison Quadrangle, Colorado




                                                   Remotely sensed
                                                   data are a special
                                                   type of raster data.




                                                 Figure 16
    Geographic Information System                              8/20/2010




                                                                                 14
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Other forms of raster data


                                                                  Scanned
                                    Aerial                        maps
                                    photos




                                    Pictures

                                                 Converted data

                                     Figure 17
    Geographic Information System                                 8/20/2010




Vector Data Model
 Treats geographic space as populated by discrete objects, which
 have identifiable b d i
 h id tifi bl boundaries or spatial extent
                                    ti l t t
 Each object in the real-world is represented as either point, line,
 or polygon features.
 characterized by the use of sequential points or vertices to define
 a linear segment, each vertex consisting of an X coordinate and
 a Y coordinate.
 Useful for representing discrete objects such as roads, buildings,
 rivers, boundaries, etc.


    Geographic Information System                                 8/20/2010




                                                                                    15
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Vector Data Model




    Vector Data Model              Raster Data Model


   Geographic Information System                       8/20/2010




Vector Data Model




   Geographic Information System                       8/20/2010




                                                                         16
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Vector data model
The vector is
composed of a
     p
string of points,
each represented
by their exact
spatial
coordinates.


                                  Points are represented by their
                                  coordinates.
  Geographic Information System                                     8/20/2010




     Vector data model




    Lines are defined as a string of x- and y- coordinates.
  Geographic Information System                                     8/20/2010




                                                                                      17
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       Vector data model




 Areas are a string of x- and y-coordinates that terminate at the origin.

    Geographic Information System                                8/20/2010




Types of Vector Data Model
       1. Simple (spaghetti) data model
             p (pg         )
       2. Topologic data model




    Geographic Information System                                8/20/2010




                                                                                   18
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Simple (Spaghetti) Data Model
    a one-for-one translation of the analog map
    Features (lines and polygons) may overlap
    No relationships between features/entities
    Generally used where analysis is not important (eg.
    Plotting)
    each entity is a single, logical record in the computer,
    coded as variable l th strings of x- and y- coordinate
       d d       i bl length t i        f      d       di t
    pairs
    Examples are digitized maps and CAD data

    Geographic Information System                       8/20/2010




Simple (Spaghetti) Data Model
        g
 Advantages:
        Easy to create and store
        Can be retrieved and rendered on screen
        very quickly




    Geographic Information System                       8/20/2010




                                                                          19
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Simple (Spaghetti) Data Model
 Disadvantages:
        Spatial analysis (eg. Shortest path network analysis)
        cannot be performed easily due to lack of any
        connectivity relationships
        Redundant since the boundary segment between two
        polygons can be stored twice (once for each feature)
        Inflexible since it is difficult to dissolve common
        boundaries when joining polygons
        Cannot be used in certain applications such as land
        management because of overlapping features
     Geographic Information System                       8/20/2010




Topological Data Model
     spaghetti model but with topology!
     Essentially just a simple d model structured using
     E             ll       l data d l                 d
     topologic rules
     often referred to as an intelligent data structure
     because spatial relationships between geographic
     features are easily derived when using them
     Most commonly used variants is the arc node data
                                             arc-node
     model
     uses nodes (intersection of two lines; end point of
     line, and any point on line) and arcs (line connecting
     two nodes) System
     Geographic Information                             8/20/2010




                                                                           20
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Topological Data Model Components
Point (vertex) – where a line originates or terminates
Node – where a link originates or terminates
Line – a segment between two points (vertices)
Link (or arc or chain) – a connection between two nodes
     – may consist of several lines which are joined at points
    (vertices)
     – can only originate, terminate or be connected at nodes
Polygon (area) – composed of links
       – adjacent polygons have only one link between them
    Geographic Information System                      8/20/2010




    Geographic Information System                      8/20/2010




                                                                         21
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Elements of Topological Relationship
 Connectivity
    Information about linkages among spatial objects
    I f     ti    b t li k                ti l bj t
    Keep track of which links are connected at a node
 Adjacency
    Information about neighbourhood among spatial objects
    A link can determine the polygon to its left and its right
 Containment
    Information about inclusion of one spatial object within
    another spatial object
    What nodes and links and other polygons are within a
    polygon
     Geographic Information System                      8/20/2010




Elements of Topological Relationship




     Geographic Information System                      8/20/2010




                                                                          22
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Topological Map
    Explains the concept of topological relationships
    A map that contains explicit topological information on
    top of the geometric information expressed in coordinates
    (Masry and Lee, 1988)
    All spatial entities are decomposed and represented in the
    form of the three basic graphical elements (point, line,
    polygon)



   Geographic Information System                      8/20/2010




Concept of a Topological Map




   Geographic Information System                      8/20/2010




                                                                        23
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Geographic Information System   8/20/2010




Geographic Information System   8/20/2010




                                                  24
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   Geographic Information System                 8/20/2010




Arc-node Data Model
   Stores graphical elements rather than graphical
   entities
   Explicitly stores spatial relationships between the
   graphical elements, and relationships between the
   arcs and respective nodes
   Stored t l i l l ti hi ll
   St d topological relationships allow graphical
                                                hi l
   entities to be constructed using the basic graphical
   elements
   Geographic Information System                 8/20/2010




                                                                   25
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Arc-node Data Model




   Geographic Information System          8/20/2010




Network Data Model
   A special type of topologic model
      p       yp       p g
   Shows how lines connect with each other at
   nodes
   Applications:
     Load analysis over an electric network
               y
     Vehicle routing over a street network
     Pollution tracing over a river network
   Geographic Information System          8/20/2010




                                                            26
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Use of Topological Relationship
 Areas in GIS wherein topological relationships are
    applied are:
 1. Data input and representation
 2. Spatial search
 3. Construction of complex spatial objects from
    basic graphical elements
    b          h l l
 4. Integrity checks in database creation


    Geographic Information System                  8/20/2010




Data input and representation
     Creating a topologic geographic database is
     extremely time-consuming and error-prone
     Possible to adopt the spaghetti digitization for
     graphical data input and then later on let the
     computer refine and structure them using
     topological relationships



    Geographic Information System                  8/20/2010




                                                                     27
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Data input and representation




   Geographic Information System                               8/20/2010




Spatial search
    GIS uses topological relationships to perform spatial
    searches efficiently
         h ffi i tl
    For example, the land parcels surrounding a particular
    parcel can be located immediately using adjacency
    information
        Polygons that share a common boundary with the parcel of
        interest are identified
        Their IDs are used to retrieve the descriptive data from the
        database


   Geographic Information System                               8/20/2010




                                                                                 28
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Spatial search




   Geographic Information System                   8/20/2010




Construction of complex spatial objects
    Complex objects are represented as complex
    polygons in a geographic d b
      l                  h database
    Two types of complex polygons are:
        Those containing one or more holes (or
        islands/enclaves) constructed from vector lines
        using topological information
        Those d
        Th made up of two or more polygons th t are not
                         ft             l      that     t
        physically connected constructed using a common
        identifier

   Geographic Information System                   8/20/2010




                                                                     29
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Construction of complex spatial objects




    Geographic Information System                  8/20/2010




Integrity checks
     Graphical data must be topologically cleaned
     before using them in GIS applications
         Must not contain any topological errors
     During topology building, the computer detects
     topological errors and marks them immediately
     Operator can then decide what to do with the
                                hat       ith
     errors


    Geographic Information System                  8/20/2010




                                                                     30
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Integrity checks




    Geographic Information System                                         8/20/2010




Triangulated Irregular Network (TIN) Data
Model
    Creates and represents surfaces in GIS
    Surface i
    S f is represented as contiguous, non-overlapping t i l
                       t d         ti                l i triangles
    Size of triangles may be adjusted to reflect the relief of the surface being
    modelled
        Larger triangles for relatively flat terrain
        Smaller triangles for rugged terrain
    Manages information about the nodes that comprise each triangle and the
    neighbors of each triangle
    Applications:
        Volumetric calculations for road design
        Drainage studies
        Landslide studies
    Geographic Information System                                         8/20/2010




                                                                                            31
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Triangulated Irregular Network (TIN) Data
Model




    Geographic Information System                 8/20/2010




Advantages of TIN Data Model
    Incorporates original sample points so the accuracy
    of the model can be checked
    Efficient way of storing surface representations
    because the size of triangles can be varied according
    to the terrain
    Data t t
    D t structure makes it easy t calculate elevation,
                       k           to l l t l ti
    slope, aspect, and line-of-sight between points

    Geographic Information System                 8/20/2010




                                                                    32
8/20/2010




Vector File Formats
  GDF/DIME (Geographic Base File/Dual Independent Map Encoding) – originally used
  for storing street maps
  TIGER (Topologically Integrated Geographic Encoding and Referencing System) –
  improvement for GDF/DIME
  DLG (Digital Line Graphs) – USGS topographic maps
  AutoCAD DXF (Data Exchange Format) – widely used as an export format in many GIS
  IGDS (Intergraph Design System) – widely used in mapping
  ArcInfo Coverage – stores vector graphical data using topological structure explicitly
  defining spatial relationships
  ArcInfo E00 – export format of ArcInfo
                     p
  Shapefiles – format of ArcView GIS; defines geometry and attribute of geographically
  referenced objects using the main file, index file and database table
  CGM (Computer Graphics Metafile) – ISO standard for vector data format; widely used
  in PC-based computer graphics applications
  Page Description Languages (PDL’s)
      Geographic Information System                                           8/20/2010




Considerations in choosing between
Raster and Vector

  source and type of data
  analytical procedures to be used
  intended use of data




      Geographic Information System                                           8/20/2010




                                                                                                 33
8/20/2010




Raster vs. Vector
  Raster Data Model
  Advantages:
   Simple data structure
   Easy to conceptualize space representation
   Easy data collection and processing
   Easy and efficient overlaying
   High spatial variability is efficiently represented
   Easier and efficient for surface modelling, wherein individual
   features are not that important
   Allows easy integration of i
   All          i        i     f image d ( lli remotely-sensed,
                                        data (satellite,   l       d
   etc.)
   Pixel values can be modified individually or in a group by using a
   palette
   Simple for own programming
     Geographic Information System                             8/20/2010




Raster vs. Vector
  Raster Data Model
  Disadvantages:
   Do not provide precise locational and area computation
   information due to grid cells
   Requires large storage capacity, based on size and number
   of colors
   No topological processing
   Limited tt ib t data handling
   Li it d attribute d t h dli
   Inefficient projection transformations
   Loss of information when using large cells
   “Blocky” appearance when image is viewed in detail
     Geographic Information System                             8/20/2010




                                                                                 34
8/20/2010




Raster vs. Vector
  Vector Data Model
  Advantages:
   Far more efficient in storage than grids Compact data
   structure
   Provides precise locational information of points
   Can represent point, line, and area features very accurately,
   leading to more accurate measurements and map outputs
   Topological models enable many types of analysis
   Sophisticated attribute data handling
   Efficient for network analysis
   Efficient projection transformation
   Work well with pen and light-plotting devices and tablet digitizers

     Geographic Information System                             8/20/2010




Raster vs. Vector
  Vector Data Model

  Disadvantages:
   Complex data structure
   Difficult overlay operations
   High spatial variability is inefficiently represented
   Not compatible with RS imagery
   Expensive data collection
   Not good at continuous coverages or plotters that fill
   areas
     Geographic Information System                             8/20/2010




                                                                                 35
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Representation of a line using raster
and vector model




Raster representation                      Vector representation



     Geographic Information System                           8/20/2010




 Raster vs. Vector
                                      Raster             Vector
Data collection               Rapid              Slow
Data volume                   Large              Small
Graphic treatment             Average            Good
Data structure                Simple             Complex
Area analysis                 Good               Average
Network analysis              Poor               Good
Generalization                Simple             Complex


     Geographic Information System                           8/20/2010




                                                                               36
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Integrating raster and vector data




    Geographic Information System    8/20/2010




                                                       37

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GIS - Lecture 4

  • 1. 8/20/2010 GIS Data Structures and Models GE 517 – Geographic Information Systems Engr. Ablao Digital Geographic Data Numerical representations that d ib real-world f N i l i h describe l ld features and phenomena Must be encoded in digital form and organized as a geographic database to be useful to GIS This digital database creates our perception of the real world Geographic Information System 8/20/2010 1
  • 2. 8/20/2010 Conventional Data vs. Digital Geographic Data Conventional Data: Paper map Static representation Represents a general-purpose snapshot of the real world at a given time only Digital Geographic Data: Dynamic representation Allows a range of functions for storing processing analyzing storing, processing, analyzing, and visualizing spatial data Has tools that allow users to interact with the data to meet their objectives Geographic Information System 8/20/2010 FIGURE 1. Bringing real-world data into a GIS or a map requires several abstractions and/or conversions of the original data. (Freely adapted from Bernhardsen, 1992). Physical Real world Map / → → Data model → Database → reality model Report Actual Entity Object Object Symbol, line, phenomenon text • Properties • Type • Type • Type • Connection • Attributes • Attributes • Attributes s • Relationship • Relationship • Relationship • Geometry • Geometry • Quality • Quality Geographic Information System 8/20/2010 2
  • 3. 8/20/2010 Figure 2 Geographic Information System 8/20/2010 What is a Data Model? The h Th heart of any GIS f A way of representing data A set of constructs for describing and representing selected aspects of the real world in a computer l ld computer. Longley, Goodchild, Maguire & Rhind Geographic Information System 8/20/2010 3
  • 4. 8/20/2010 Geographic Data Models REAL WORLD Field-based model Object-based model Raster data model Vector data model Figure 3 Geographic Information System 8/20/2010 Raster and Vector Data Models Geographic Information System 8/20/2010 Figure 4 4
  • 5. 8/20/2010 Raster Data Model Treats geographic space as populated by one or more spatial phenomena, phenomena which vary continuously over space and having no obvious boundaries. Best used to represent geographic features that are continuous over a large area (e.g. soil type, vegetation, etc.) Uses an array of rectangular cells/pixels/grids to represent real-world objects. real world objects Each cell is defined by a coordinate location and an attribute that identifies the feature. Cell’s linear dimensions defines the spatial resolution Geographic Information System 8/20/2010 Raster Data Model Figure 5 Geographic Information System 8/20/2010 5
  • 6. 8/20/2010 Raster Data Model Figure 6 Geographic Information System 8/20/2010 Raster Data Model Figure 7 Geographic Information System 8/20/2010 6
  • 7. 8/20/2010 A point is represented as a single cell. well tree rock Figure 8 Geographic Information System 8/20/2010 A line is represented as a string of cells. river road canal Figure 9 Geographic Information System 8/20/2010 7
  • 8. 8/20/2010 An area is a contiguous block of cells. grassland barn forest Figure 10 Geographic Information System 8/20/2010 Raster Data Model The cell size determines the resolution of the data. The grid cell has an extent. When we map features onto the grid, there is sometimes an imperfect fit. Each grid cell can usually be “owned” by one feature, that is, the one whose attribute it holds (mixed pixels). Geographic Information System 8/20/2010 8
  • 9. 8/20/2010 Figure 11 Geographic Information System 8/20/2010 Figure 12 Geographic Information System 8/20/2010 9
  • 10. 8/20/2010 Raster Data Compression A single raster file usually contains several million grid cells l d to a very l ll leading large d f l data file Therefore, data compression is very important in the digital representation of raster data! Geographic Information System 8/20/2010 Raster Data Compression Based on the realization that cells representing the same ffeature type have identical values and that they h d l l d h h tend to be spatially clumped Examples of raster data compression techniques are: Run length encoding Wavelet compression Geographic Information System 8/20/2010 10
  • 11. 8/20/2010 Raster Data Compression Run length encoding g g Adjacent cells along a row having the same value are treated as a group known as a ‘run’ instead of storing the similar cell values individually, the value is just stored once together with the number of cells that comprise the run b f ll th t i th Geographic Information System 8/20/2010 Run length encoding Figure 13 Geographic Information System 8/20/2010 11
  • 12. 8/20/2010 Raster Data Compression Wavelet Compression p More efficient compression Can achieve a ratio between 15:1 to 20:1 for 8-bit gray scale images and between 30:1 to 40:1 for 24- bit color images Geographic Information System 8/20/2010 Raster File Formats BMP (bitmaps) – used by graphics in Microsoft Windows applications; no p compression DIB (Device Independent Bitmaps) GIF (Graphical Interchange Format) – widely used for images to be used on the World Wide Web TIFF (Tagged Image File Format) – non-proprietary, system-independent JPEG (Joint Photographic Experts Group) – primarily for storage of photographic images and for the World Wide Web GeoTIFF – extension of the TIFF format that contains georeferencing information PNG (Portable Network Graphics) – patent-free; intended to replace the GIF format PCX – supported by many image scanners MrSID (Multi-resolution Seamless Image Database) GRID – proprietary format used by ESRI in ArcInfo and ArcView GIS Geographic Information System 8/20/2010 12
  • 13. 8/20/2010 Quadtree Data Model A hierarchical field-based model that uses grid cells of variable sizes f i bl i Uses finer subdivisions in areas where finer detail occurs Geographic space is divided by the process of recursive decomposition More difficult compared to the simple raster model Geographic Information System 8/20/2010 Quadtree Data Model Figure 14 Geographic Information System 8/20/2010 13
  • 14. 8/20/2010 Digital Elevation Model A raster-based representation of surface Figure 15 Geographic Information System Southwest Corner of the 8/20/2010 Morrison Quadrangle, Colorado Remotely sensed data are a special type of raster data. Figure 16 Geographic Information System 8/20/2010 14
  • 15. 8/20/2010 Other forms of raster data Scanned Aerial maps photos Pictures Converted data Figure 17 Geographic Information System 8/20/2010 Vector Data Model Treats geographic space as populated by discrete objects, which have identifiable b d i h id tifi bl boundaries or spatial extent ti l t t Each object in the real-world is represented as either point, line, or polygon features. characterized by the use of sequential points or vertices to define a linear segment, each vertex consisting of an X coordinate and a Y coordinate. Useful for representing discrete objects such as roads, buildings, rivers, boundaries, etc. Geographic Information System 8/20/2010 15
  • 16. 8/20/2010 Vector Data Model Vector Data Model Raster Data Model Geographic Information System 8/20/2010 Vector Data Model Geographic Information System 8/20/2010 16
  • 17. 8/20/2010 Vector data model The vector is composed of a p string of points, each represented by their exact spatial coordinates. Points are represented by their coordinates. Geographic Information System 8/20/2010 Vector data model Lines are defined as a string of x- and y- coordinates. Geographic Information System 8/20/2010 17
  • 18. 8/20/2010 Vector data model Areas are a string of x- and y-coordinates that terminate at the origin. Geographic Information System 8/20/2010 Types of Vector Data Model 1. Simple (spaghetti) data model p (pg ) 2. Topologic data model Geographic Information System 8/20/2010 18
  • 19. 8/20/2010 Simple (Spaghetti) Data Model a one-for-one translation of the analog map Features (lines and polygons) may overlap No relationships between features/entities Generally used where analysis is not important (eg. Plotting) each entity is a single, logical record in the computer, coded as variable l th strings of x- and y- coordinate d d i bl length t i f d di t pairs Examples are digitized maps and CAD data Geographic Information System 8/20/2010 Simple (Spaghetti) Data Model g Advantages: Easy to create and store Can be retrieved and rendered on screen very quickly Geographic Information System 8/20/2010 19
  • 20. 8/20/2010 Simple (Spaghetti) Data Model Disadvantages: Spatial analysis (eg. Shortest path network analysis) cannot be performed easily due to lack of any connectivity relationships Redundant since the boundary segment between two polygons can be stored twice (once for each feature) Inflexible since it is difficult to dissolve common boundaries when joining polygons Cannot be used in certain applications such as land management because of overlapping features Geographic Information System 8/20/2010 Topological Data Model spaghetti model but with topology! Essentially just a simple d model structured using E ll l data d l d topologic rules often referred to as an intelligent data structure because spatial relationships between geographic features are easily derived when using them Most commonly used variants is the arc node data arc-node model uses nodes (intersection of two lines; end point of line, and any point on line) and arcs (line connecting two nodes) System Geographic Information 8/20/2010 20
  • 21. 8/20/2010 Topological Data Model Components Point (vertex) – where a line originates or terminates Node – where a link originates or terminates Line – a segment between two points (vertices) Link (or arc or chain) – a connection between two nodes – may consist of several lines which are joined at points (vertices) – can only originate, terminate or be connected at nodes Polygon (area) – composed of links – adjacent polygons have only one link between them Geographic Information System 8/20/2010 Geographic Information System 8/20/2010 21
  • 22. 8/20/2010 Elements of Topological Relationship Connectivity Information about linkages among spatial objects I f ti b t li k ti l bj t Keep track of which links are connected at a node Adjacency Information about neighbourhood among spatial objects A link can determine the polygon to its left and its right Containment Information about inclusion of one spatial object within another spatial object What nodes and links and other polygons are within a polygon Geographic Information System 8/20/2010 Elements of Topological Relationship Geographic Information System 8/20/2010 22
  • 23. 8/20/2010 Topological Map Explains the concept of topological relationships A map that contains explicit topological information on top of the geometric information expressed in coordinates (Masry and Lee, 1988) All spatial entities are decomposed and represented in the form of the three basic graphical elements (point, line, polygon) Geographic Information System 8/20/2010 Concept of a Topological Map Geographic Information System 8/20/2010 23
  • 24. 8/20/2010 Geographic Information System 8/20/2010 Geographic Information System 8/20/2010 24
  • 25. 8/20/2010 Geographic Information System 8/20/2010 Arc-node Data Model Stores graphical elements rather than graphical entities Explicitly stores spatial relationships between the graphical elements, and relationships between the arcs and respective nodes Stored t l i l l ti hi ll St d topological relationships allow graphical hi l entities to be constructed using the basic graphical elements Geographic Information System 8/20/2010 25
  • 26. 8/20/2010 Arc-node Data Model Geographic Information System 8/20/2010 Network Data Model A special type of topologic model p yp p g Shows how lines connect with each other at nodes Applications: Load analysis over an electric network y Vehicle routing over a street network Pollution tracing over a river network Geographic Information System 8/20/2010 26
  • 27. 8/20/2010 Use of Topological Relationship Areas in GIS wherein topological relationships are applied are: 1. Data input and representation 2. Spatial search 3. Construction of complex spatial objects from basic graphical elements b h l l 4. Integrity checks in database creation Geographic Information System 8/20/2010 Data input and representation Creating a topologic geographic database is extremely time-consuming and error-prone Possible to adopt the spaghetti digitization for graphical data input and then later on let the computer refine and structure them using topological relationships Geographic Information System 8/20/2010 27
  • 28. 8/20/2010 Data input and representation Geographic Information System 8/20/2010 Spatial search GIS uses topological relationships to perform spatial searches efficiently h ffi i tl For example, the land parcels surrounding a particular parcel can be located immediately using adjacency information Polygons that share a common boundary with the parcel of interest are identified Their IDs are used to retrieve the descriptive data from the database Geographic Information System 8/20/2010 28
  • 29. 8/20/2010 Spatial search Geographic Information System 8/20/2010 Construction of complex spatial objects Complex objects are represented as complex polygons in a geographic d b l h database Two types of complex polygons are: Those containing one or more holes (or islands/enclaves) constructed from vector lines using topological information Those d Th made up of two or more polygons th t are not ft l that t physically connected constructed using a common identifier Geographic Information System 8/20/2010 29
  • 30. 8/20/2010 Construction of complex spatial objects Geographic Information System 8/20/2010 Integrity checks Graphical data must be topologically cleaned before using them in GIS applications Must not contain any topological errors During topology building, the computer detects topological errors and marks them immediately Operator can then decide what to do with the hat ith errors Geographic Information System 8/20/2010 30
  • 31. 8/20/2010 Integrity checks Geographic Information System 8/20/2010 Triangulated Irregular Network (TIN) Data Model Creates and represents surfaces in GIS Surface i S f is represented as contiguous, non-overlapping t i l t d ti l i triangles Size of triangles may be adjusted to reflect the relief of the surface being modelled Larger triangles for relatively flat terrain Smaller triangles for rugged terrain Manages information about the nodes that comprise each triangle and the neighbors of each triangle Applications: Volumetric calculations for road design Drainage studies Landslide studies Geographic Information System 8/20/2010 31
  • 32. 8/20/2010 Triangulated Irregular Network (TIN) Data Model Geographic Information System 8/20/2010 Advantages of TIN Data Model Incorporates original sample points so the accuracy of the model can be checked Efficient way of storing surface representations because the size of triangles can be varied according to the terrain Data t t D t structure makes it easy t calculate elevation, k to l l t l ti slope, aspect, and line-of-sight between points Geographic Information System 8/20/2010 32
  • 33. 8/20/2010 Vector File Formats GDF/DIME (Geographic Base File/Dual Independent Map Encoding) – originally used for storing street maps TIGER (Topologically Integrated Geographic Encoding and Referencing System) – improvement for GDF/DIME DLG (Digital Line Graphs) – USGS topographic maps AutoCAD DXF (Data Exchange Format) – widely used as an export format in many GIS IGDS (Intergraph Design System) – widely used in mapping ArcInfo Coverage – stores vector graphical data using topological structure explicitly defining spatial relationships ArcInfo E00 – export format of ArcInfo p Shapefiles – format of ArcView GIS; defines geometry and attribute of geographically referenced objects using the main file, index file and database table CGM (Computer Graphics Metafile) – ISO standard for vector data format; widely used in PC-based computer graphics applications Page Description Languages (PDL’s) Geographic Information System 8/20/2010 Considerations in choosing between Raster and Vector source and type of data analytical procedures to be used intended use of data Geographic Information System 8/20/2010 33
  • 34. 8/20/2010 Raster vs. Vector Raster Data Model Advantages: Simple data structure Easy to conceptualize space representation Easy data collection and processing Easy and efficient overlaying High spatial variability is efficiently represented Easier and efficient for surface modelling, wherein individual features are not that important Allows easy integration of i All i i f image d ( lli remotely-sensed, data (satellite, l d etc.) Pixel values can be modified individually or in a group by using a palette Simple for own programming Geographic Information System 8/20/2010 Raster vs. Vector Raster Data Model Disadvantages: Do not provide precise locational and area computation information due to grid cells Requires large storage capacity, based on size and number of colors No topological processing Limited tt ib t data handling Li it d attribute d t h dli Inefficient projection transformations Loss of information when using large cells “Blocky” appearance when image is viewed in detail Geographic Information System 8/20/2010 34
  • 35. 8/20/2010 Raster vs. Vector Vector Data Model Advantages: Far more efficient in storage than grids Compact data structure Provides precise locational information of points Can represent point, line, and area features very accurately, leading to more accurate measurements and map outputs Topological models enable many types of analysis Sophisticated attribute data handling Efficient for network analysis Efficient projection transformation Work well with pen and light-plotting devices and tablet digitizers Geographic Information System 8/20/2010 Raster vs. Vector Vector Data Model Disadvantages: Complex data structure Difficult overlay operations High spatial variability is inefficiently represented Not compatible with RS imagery Expensive data collection Not good at continuous coverages or plotters that fill areas Geographic Information System 8/20/2010 35
  • 36. 8/20/2010 Representation of a line using raster and vector model Raster representation Vector representation Geographic Information System 8/20/2010 Raster vs. Vector Raster Vector Data collection Rapid Slow Data volume Large Small Graphic treatment Average Good Data structure Simple Complex Area analysis Good Average Network analysis Poor Good Generalization Simple Complex Geographic Information System 8/20/2010 36
  • 37. 8/20/2010 Integrating raster and vector data Geographic Information System 8/20/2010 37