Geographical Information Systems

Geog 481/506                                                                                                  Tu Th 3:30-4:50pm
Fall 2011                                                                                                          Fillmore 170

Instructor: Ling Bian                                                                                          LabA: Tue:   5-6:20pm, W145, Chunyuan diao
Office: 120 Wilkeson                                                                                        LabB: Thur:  6:30-7:50pm, W145, Tong Sun
Office hours: Tu Th 2-3pm  or by appt.                                                             LabC: Fri:  10-11:20am, W145, Tong Sun

Data models and Data Structures

1. Components of geographic data
(1) Spatial locations 
    Spatial locations
            Specified with reference to a common coordinate system

            points
            lines
            polygons
            volumes

(2) Attributes
            Nominal
            Ordinal
            Interval
            Ratio

(3) Topology
            Spatial relationship between geographic features
            Adjacency, containment, connectivity, and so on

(4) Time
 

2. Spatial data models
    Real world entities and spatial entities

    The object and field view

(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.

(2) Raster
    A matrix consists of regular grid cells.
    Positions are defined by column and row numbers.
    Each cell has a single value.

(3) Advantages and disadvantages
      Raster
       Ad:   It is a simple data model
               Overlay operation can be easily implemented
               High spatial variation is efficiently represented
 
      Disad: It is less compact
                Topological relationships are difficult to represent
                The output is less aesthetically pleasing

      Vector
        Ad:  It is a compact data model
                It is efficient in coding topology
                The output closely approximate hand-drawn maps

        Disad: It is a complex data model
                  Overlay operations are difficult to implement
                  The representation of high variation is inefficient

3. Raster data structures
(1) Run length
    Attribute1, number of cells in the run; attribute2, number of cells in the run.
 
 
 
 

(2) Quadtrees
    Subdividing a region into quadrants until each quadrant contains only one class - variable resolution.
 
 
 
 

4. Vector data structures
(1) Non-topological structure
    There is no topology and shared boundaries are recorded twice.
 
 
 
 

(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

    The arc-node model
    Arc - a line stating 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

    Polygon topology table
    Node topology table
    Arc topology table

    Arc coordinate data table
 
 

5. Readings: Chpt 3.