Geographic Information SystemsGeographic Information Systems

GIS Data Models

1. Components of Geographic 1. Components of Geographic DataData

Spatial locations Attributes Topology Time

Components – Spatial LocationsComponents – Spatial Locations

Specified with reference to a common coordinate system

        points      lines     polygons     volumes

grids

Components - AttributesComponents - 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., , e, -96.345, …

ComponentsComponents

Topology

Spatial relationship between geographic features

  Adjacency

Containment

Connectivity

etc.

Time

Presently treated as an attribute

2. Data Models2. Data Models

Real world entities and spatial entities

Data Models – Objects and FieldsData Models – Objects and Fields

Objects and Field View

Objects

- Are discrete or have identifiable boundaries

- Have attributes

Field

- Varies continuously

- Has no obvious boundaries

Data ModelsData Models

Vector data model Raster data model

courtesy: Mary Ruvane, http://ils.unc.edu/

Data ModelsData Models

Data Models - (1) VectorData 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.

Epidemiological Studies- Disease TrackingEpidemiological Studies- Disease Tracking

Incidence of Viruses

ESRI, GE SmallWorld

using the land-cover map and field snail survey including 93 positive snail sites and 800 non-positive snail sites

0 3,300 6,6001,650 Meters

±

Positive snail

0.00 - 0.03

0.04 - 0.13

0.14 - 0.23

0.24 - 0.33

0.34 - 0.60

Lake

Positive Snail Prediction of Xichang Study Site

Data Models - (2) RasterData Models - (2) Raster

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

Data Models - (2) RasterData Models - (2) Raster Data consists 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

Data Models - (2) RasterData Models - (2) Raster

2 Data Model2 Data Model

(3) Advantages and Disadvantages(3) Advantages and Disadvantages Raster Vector

Ad and Disad - RasterAd 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

Ad and Disad - VectorAd 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

ReadingsReadings

Chapter 3

Geographic Information SystemsGeographic Information Systems

GIS Data Structures

GIS Data StructuresGIS Data Structures

Raster data structures

- Run length     - Quadtrees

Vector data structures

- Non-topological structure

- Topological structure      

3 Raster Data Structures3 Raster Data Structures(1) Run Length(1) Run Length

Data are recorded in the order of

Attribute1, number of cells in the run;

Attribute2, number of cells in the run; …  

 

3 Raster (1) Run Length3 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

…

11 11 11 11 3 3 3 3

11 11 11 11 3 3 3 3

11 11 11 11 3 3 3 3

11 11 11 11 3 3 3 3

11 11 11 3 5 5 5 5

11 11 5 5 5 5 5 5

11 11 5 5 5 5 5 5

11 11 5 5 5 5 5 5

3 Raster Data Structures3 Raster Data Structures(2) Quadtrees(2) Quadtrees

Subdividing a region into quadrants until each quadrant contains only one class - variable resolution

 

3 (2) Quadtrees3 (2) Quadtrees

3 (2) Quadtrees3 (2) Quadtrees

4 Vector Data Structures4 Vector Data Structures(1) Non-topological Structure(1) Non-topological Structure

There is no topology and shared boundaries are recorded twice

 

4 Vector (1) Non-Topological 4 Vector (1) Non-Topological StructureStructure

Storing coordinates of

two polygons: and

Polygon

x1, y1; x2,y2; x3,y3; …. x1,y1.

Polygon

x1, y1; x2,y2; x3,y3; …. x1,y1.

(x1, y1) (x1, y1)

(x2, y2) (x2, y2)

A

B

1 2

1 2

1

2

4 Vector (2) Topological Structure4 Vector (2) Topological Structure No node or 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

4 Vector (2) Topological Structure4 Vector (2) Topological StructureStoring coordinates of two polygons: and

Line 1

x1, y1; x2,y2; x3, y3; x4, y4; x5,y5; Line 2

x1, y1; x2,y2; Line 3

x1, y1; x2,y2; x3, y3; x4, y4;

Polygon Line 1, Line 2Polygon Line 2, Line 3

(x1, y1) (x1, y1)

(x2, y2) (x2, y2)

1 2

1 2

1

2

Line 1

Line 2

Line 3

4 Vector (2) Topological Structure4 Vector (2) Topological Structure Polygon = a list of lines Line = a set of coordinates

Topological- the Arc-Node ModelTopological- 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

Arc-Node ModelArc-Node Model

  Polygon topology table Node topology table Arc topology table

Arc coordinate data table

 

ReadingsReadings

Chapter 3