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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 points lines polygons volumesgrids

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Components - Attributes ► Nominal - qualitative, e.g., grasses, trees, buildings, - qualitative, e.g., grasses, trees, buildings, ► Ordinal - rankble quality, e.g., high, medium, low - rankble quality, e.g., high, medium, low ► Interval - quantitative but not ratio, e.g., 40 0 F, 80 0 F ► Ratio - absolute quantities, e.g., , e, , …

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Components - Topology ► Topology Spatial relationship between geographic features Spatial relationship between geographic features Adjacency AdjacencyContainmentConnectivityetc. ► 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 - 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, 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 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 03,3006,6001,650Meters ± 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 - 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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► World views - object - field ► Data models - vector - raster ► Data structures ► 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 - 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; … Attribute2, number of cells in the run; …

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3 Raster (1) Run Length 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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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 x 1, y 1 ; x 2,y 2 ; x 3,y 3 ; x 4,y 4 ; x 5,y 5 ; x 1,y 1. Polygon x 1, y 1 ; x 6,y 6 ; x 7,y 7 ; x 2,y 2 ; x 1,y 1. (x1, y1) (x2, y2) A B

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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 x 1, y 1 ; x 2,y 2 ; x 3, y 3 ; x 4, y 4 ; x 5,y 5 ; Line 2 x 1, y 1 ; x 5,y 5 ; Line 3 x 5, y 5 ; x 6,y 6 ; x 7, y 7 ; x 1, y 1 ; Polygon Line 1, Line 2 Polygon Line 2, Line 3 (x1, y1) (x5, y5) Line 1 Line 2 Line 3

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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 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 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 Polygon topology table Node topology table Arc topology table Arc coordinate data table Arc coordinate data table

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Readings Chapter 3 Chapter 3

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