Geographic Information Systems

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, -96.345, …

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, http://ils.unc.edu/

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 ESRI, GE SmallWorld

11. Positive Snail Prediction of Xichang Study Site using the land-cover map and field snail survey including 93 positive snail sites and 800 non-positive snail sites Positive snail 0.00 - 0.03 0.04 - 0.13 0.14 - 0.23 0.24 - 0.33 0.34 - 0.60 Lake ± 0 1,650 3,300 6,600 Meters

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 11,4; 3,4; 11,4; 3,4; 11,4; 3,4; 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. Polygon 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 Line 3 2 (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