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Geographic Information Systems

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Presentation on theme: "Geographic Information Systems"— Presentation transcript:

1 Geographic Information Systems
GIS Data Models

2 1. Components of Geographic Data
Spatial locations Attributes Topology Time

3 Components – Spatial Locations
Specified with reference to a common coordinate system Spatial features can be represented as         points       lines      polygons      volumes grids

4 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, , …

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

6 2. Data Models Real world entities and spatial entities

7 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

8 Data Models Vector data model Raster data model
courtesy: Mary Ruvane,

9 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.

10 Epidemiological Studies- Disease Tracking
Incidence of Viruses Epidemiological Studies- Disease Tracking ESRI, GE SmallWorld

11 using the land-cover map and field snail survey including 93 positive snail sites and 800 non-positive snail sites 3,300 6,600 1,650 Meters Positive snail Lake Positive Snail Prediction of Xichang Study Site


13 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

14 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

15 Data Models - (2) Raster

16 2 Data Model (3) Advantages and Disadvantages
Raster Vector

17 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

18 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

19 Readings Chapter 3

20 Geographic Information Systems
GIS Data Structures

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

22 GIS Data Structures Raster data structures - Run length - Quadtrees
Vector data structures - Non-topological structure - Topological structure      

23 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; …  

24 3 Raster (1) Run Length 11 3 5 11,4; 3,4; 11,3; 3,1; 5,4 …

25 3 Raster Data Structures (2) Quadtrees
Subdividing a region into quadrants until each quadrant contains only one class - variable resolution

26 3 (2) Quadtrees

27 3 (2) Quadtrees

28 4 Vector Data Structures (1) Non-topological Structure
Polygons are the basis One polygon at a time, the data structure records coordinates of the outline of the polygon There is no topology, and shared boundaries are recorded twice

29 4 Vector (1) Non-Topological Structure
Storing coordinates of two polygons: and Polygon x1, y1; x2,y2; x3,y3; x4,y4; x5,y5; x1,y1. x1, y1; x6,y6; x7,y7; x2,y2; x1,y1. A (x1, y1) (x1, y1) 1 2 1 2 B (x2, y2) (x2, y2) 1 2

30 4 Vector (2) Topological Structure
Lines are the basis Polygons and nodes are defined by lines 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

31 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 2 Line 3 (x5, y5) (x5, y5) 1 2

32 4 Vector (2) Topological Structure
Polygon = a list of lines Line = a set of coordinates Point = a line with no length

33 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

34 Arc-Node Model Polygon topology table Node topology table Arc topology table Arc coordinate data table


36 Readings Chapter 3

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