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