Geographic Information Systems

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## Geographic Information Systems

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**Geographic Information Systems**GIS Data Models**1. Components of Geographic Data**• Spatial locations • Attributes • Topology • Time**Components – Spatial Locations**• Specified with reference to a common coordinate system • Spatial features can be represented as points lines polygons volumes grids**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, …**Components - Topology**• Topology Spatial relationship between geographic features Adjacency Containment Connectivity etc. • Time Presently treated as an attribute**2. Data Models**• Real world entities and spatial entities**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**Data Models**• Vector data model • Raster data model courtesy: Mary Ruvane, http://ils.unc.edu/**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.**Epidemiological Studies- Disease Tracking**Incidence of Viruses ESRI, GE SmallWorld**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**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**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**2 Data Model(3) Advantages and Disadvantages**• Raster • Vector**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**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**Readings**• Chapter 3**Geographic Information Systems**GIS Data Structures**GIS Data Structures**• World views - object - field • Data models - vector - raster • Data structures - non-topological and topological - run-length and quadtrees**GIS Data Structures**• Raster data structures - Run length - Quadtrees • Vector data structures - Non-topological structure - Topological structure**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; …**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 …**3 Raster Data Structures(2) Quadtrees**• Subdividing a region into quadrants until each quadrant contains only one class - variable resolution**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**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**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**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**4 Vector (2) Topological Structure**• Polygon = a list of lines • Line = a set of coordinates • Point = a line with no length**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**Arc-Node Model**Polygon topology table Node topology table Arc topology table Arc coordinate data table**Readings**Chapter 3