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**Geographic Information Systems**

GIS Data Models

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**1. Components of Geographic Data**

Spatial locations Attributes Topology Time

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**Components – Spatial Locations**

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

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**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, , …

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**Components - Topology Topology**

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

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2. Data Models Real world entities and spatial entities

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**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

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**Data Models Vector data model Raster data model**

courtesy: Mary Ruvane,

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**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.

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**Epidemiological Studies- Disease Tracking**

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

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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

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**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

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**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

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Data Models - (2) Raster

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**2 Data Model (3) Advantages and Disadvantages**

Raster Vector

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**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

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**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

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Readings Chapter 3

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**Geographic Information Systems**

GIS Data Structures

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**GIS Data Structures World views - object - field Data models - vector**

- raster Data structures - non-topological and topological - run-length and quadtrees

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**GIS Data Structures Raster data structures - Run length - Quadtrees**

Vector data structures - Non-topological structure - Topological structure

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**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; …

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

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

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

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3 (2) Quadtrees

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3 (2) Quadtrees

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**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

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**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

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**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

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**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

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**4 Vector (2) Topological Structure**

Polygon = a list of lines Line = a set of coordinates Point = a line with no length

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**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

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

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Readings Chapter 3

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