Geographic Information Systems

Geographic Information Geographic Information SystemsSystems

GIS Data ModelsGIS Data Models

1. Components of Geographic 1. Components of Geographic DataData

► Spatial locationsSpatial locations► AttributesAttributes► TopologyTopology

► Time Time

Components – Spatial LocationsComponents – Spatial Locations

► Specified with reference to a common Specified with reference to a common coordinate systemcoordinate system

► Spatial features can be represented as Spatial features can be represented as

pointspoints lineslines polygonspolygons volumesvolumes

gridsgrids

Components - AttributesComponents - Attributes

► Nominal Nominal - qualitative, e.g., grasses, trees, - qualitative, e.g., grasses, trees, buildings, buildings,

► Ordinal Ordinal - rankble quality, e.g., high, medium, low- rankble quality, e.g., high, medium, low

► Interval Interval

- quantitative but not ratio, e.g., 40- quantitative but not ratio, e.g., 4000F, 80F, 8000FF

► Ratio Ratio

- absolute quantities, e.g., - absolute quantities, e.g., , , ee, -96.345, …, -96.345, …

Components - TopologyComponents - Topology

► TopologyTopology

Spatial relationship between geographic Spatial relationship between geographic featuresfeatures

AdjacencyAdjacency

ContainmentContainment

ConnectivityConnectivity

etc. etc.

► TimeTime

Presently treated as an attributePresently treated as an attribute

2. Data Models2. Data Models

► Real world entities and spatial entitiesReal world entities and spatial entities

Data Models – Objects and Data Models – Objects and FieldsFields

► Object and Field ViewObject and Field View

► Objects Objects

- Are discrete or have identifiable - Are discrete or have identifiable boundariesboundaries

- Have attributes- Have attributes

► FieldsFields

- Are continuously- Are continuously

- Attributes vary across the field- Attributes vary across the field

Data ModelsData Models

► Vector data modelVector data model► Raster data modelRaster data model

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

Data Models - (1) VectorData Models - (1) Vector

► Every position has a pair of coordinates.Every position has a pair of coordinates.► Lines and polygons are constructed by Lines and polygons are constructed by connecting a series of points.connecting a series of points.

► Points, lines, and polygons are used to Points, lines, and polygons are used to represent geographic features. represent geographic features.

Epidemiological Studies- Disease Epidemiological Studies- Disease TrackingTracking

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 cellsA matrix consists of regular grid cells► Positions are defined by column and row Positions are defined by column and row numbersnumbers

► Each cell has a single value Each cell has a single value


► Data consist of mostly attributesData consist of mostly attributes► A header file stores (1) Number of rows and A header file stores (1) Number of rows and columns, (2) Cell size, and (3) Coordinates columns, (2) Cell size, and (3) Coordinates of the origin of the origin

► Coordinates of a location can be converted Coordinates of a location can be converted on the flyon the fly


2 Data Model2 Data Model(3) Advantages and (3) Advantages and

DisadvantagesDisadvantages ► RasterRaster► Vector Vector

Ad and Disad - RasterAd and Disad - Raster

► AdvantagesAdvantages

- It is a simple data model- It is a simple data model - Overlay operation can be easily - Overlay operation can be easily implementedimplemented - High spatial variation is efficiently - High spatial variation is efficiently representedrepresented

► DisadvantagesDisadvantages

- It is less compact- It is less compact- Topological relationships are difficult to - Topological relationships are difficult to

representrepresent - The output is less aesthetically pleasing - The output is less aesthetically pleasing

Ad and Disad - VectorAd and Disad - Vector

► AdvantagesAdvantages

- It is a compact data model- It is a compact data model - It is efficient in coding topology - It is efficient in coding topology

- The output closely approximate hand-- The output closely approximate hand-drawn mapsdrawn maps

► DisadvantagesDisadvantages

- It is a complex data model- It is a complex data model - Overlay operations are difficult to - Overlay operations are difficult to implementimplement - The representation of high variation is - The representation of high variation is inefficientinefficient

ReadingsReadings

► Chapter 3Chapter 3

Geographic Information Geographic Information SystemsSystems

GIS Data StructuresGIS Data Structures


► World views World views

- object- object

- field- field► Data models Data models

- vector - vector

- rasterraster► Data structures Data structures

- non-topological and topological- non-topological and topological

- run-length and quadtrees- run-length and quadtrees


► Raster data structuresRaster data structures

- Run length- Run length - Quadtrees- Quadtrees

► Vector data structures Vector data structures

- Non-topological structure- Non-topological structure

- Topological structure - Topological structure

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

► Data are recorded in the order of Data are recorded in the order of

Attribute1, number of cells in the run; Attribute1, number of cells in the run;

Attribute2, number of cells in the run; …Attribute2, number of cells in the run; …

3 Raster (1) Run Length3 Raster (1) Run Length

1111,4; ,4; 33,4;,4;

1111,4; ,4; 33,4;,4;

1111,4; ,4; 33,4;,4;

1111,4; ,4; 33,4;,4;

1111,3; ,3; 33,1; ,1; 55,4,4

……

1111 1111 1111 1111 33 33 33 33

1111 1111 1111 1111 33 33 33 33

1111 1111 1111 1111 33 33 33 33

1111 1111 1111 1111 33 33 33 33

1111 1111 1111 33 55 55 55 55

1111 1111 55 55 55 55 55 55

1111 1111 55 55 55 55 55 55

1111 1111 55 55 55 55 55 55

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

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

► Polygons are the basisPolygons are the basis► One polygon at a time, the data structure One polygon at a time, the data structure records coordinates of the outline of the records coordinates of the outline of the polygonpolygon

► There is no topology, and shared boundaries There is no topology, and shared boundaries are recorded twiceare recorded twice

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

Storing coordinates of Storing coordinates of

two polygons: and two polygons: and

Polygon Polygon

xx11, y, y11; x; x22,y,y22;; xx33,y,y33; x; x44,y,y44; x; x55,y,y55; x; x11,y,y11..

PolygonPolygon

xx11, y, y11; x; x66,y,y66; x; x77,y,y77; ; xx22,y,y22;; xx11,y,y11..

(x1, y1) (x1, y1)

(x2, y2) (x2, y2)

A

B

1 2

1 2

1

2

4 Vector (2) Topological 4 Vector (2) Topological StructureStructure

► Lines are the basisLines are the basis► Polygons and nodes are defined by linesPolygons and nodes are defined by lines► No line segment is duplicatedNo line segment is duplicated► Line segments and nodes can be referenced Line segments and nodes can be referenced to more than one polygonsto more than one polygons

► All polygons have unique identifiersAll polygons have unique identifiers► Island and hole polygons can be uniquely Island and hole polygons can be uniquely represented represented


Storing coordinates of two polygons: and Storing coordinates of two polygons: and

Line 1Line 1

xx11, y, y11; x; x22,y,y22; x; x33, y, y33; x; x44, y, y44; x; x55,y,y55; ; Line 2Line 2

xx11, y, y11; x; x55,y,y55; ; Line 3Line 3

xx55, y, y55; x; x66,y,y66; x; x77, y, y77; x; x11, y, y11;;

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

(x1, y1) (x1, y1)

(x5, y5) (x5, y5)

1 2

1 2

1

2

Line 1

Line 2

Line 3


► Polygon = a list of linesPolygon = a list of lines► Line = a set of coordinatesLine = a set of coordinates► Point = a line Point = a line

with no lengthwith no length

Topological- the Arc-Node Topological- the Arc-Node

ModelModel ► Arc - a line starting and ending at a nodeArc - a line starting and ending at a node► Node - intersection point where two or Node - intersection point where two or more arcs meet more arcs meet

or it is a dead endor it is a dead end► Polygon - a closed chain of arcs Polygon - a closed chain of arcs

Arc-Node ModelArc-Node Model

Polygon topology table Polygon topology table Node topology table Node topology table Arc topology table Arc topology table

Arc coordinate data tableArc coordinate data table

ReadingsReadings

Chapter 3Chapter 3

Documents

Geographic Information Systems