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Geographic Information Systems GIS Data Models Slide 2 1. Components of Geographic Data Spatial locations Attributes Topology Time Slide 3 Components Spatial Locations Specified with reference to a common coordinate system points lines polygons volumes grids Slide 4 Components - Attributes Nominal - qualitative, e.g., grasses, trees, buildings, Ordinal - 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, -96.345, Slide 5 Components Topology Spatial relationship between geographic features Adjacency Containment Connectivity etc. Time Presently treated as an attribute Slide 6 2. Data Models Real world entities and spatial entities Slide 7 Data Models Objects and Fields Objects and Field View Objects - Are discrete or have identifiable boundaries - Have attributes Field - Varies continuously - Has no obvious boundaries Slide 8 Data Models Vector data model Raster data model courtesy: Mary Ruvane, http://ils.unc.edu/ Slide 9 Data Models Slide 10 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. Slide 11 Epidemiological Studies- Disease Tracking Incidence of Viruses ESRI, GE SmallWorld Slide 12 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 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 Slide 13 Slide 14 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 Slide 15 Data 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 Slide 16 Data Models - (2) Raster Slide 17 2 Data Model (3) Advantages and Disadvantages Raster Vector Slide 18 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 Slide 19 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 Slide 20 Readings Chapter 3 Slide 21 Geographic Information Systems GIS Data Structures Slide 22 Raster data structures - Run length - Quadtrees Vector data structures - Non-topological structure - Topological structure Slide 23 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; Slide 24 3 Raster (1) Run Length 11,4; 3,4; 11,3; 3,1; 5,4 11 3333 3333 3333 3333 35555 555555 555555 555555 Slide 25 3 Raster Data Structures (2) Quadtrees Subdividing a region into quadrants until each quadrant contains only one class - variable resolution Slide 26 3 (2) Quadtrees Slide 27 Slide 28 4 Vector Data Structures (1) Non-topological Structure There is no topology and shared boundaries are recorded twice Slide 29 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 1,y 1. Polygon x 1, y 1 ; x 2,y 2 ; x 3,y 3 ; . x 1,y 1. (x1, y1) (x2, y2) A B 1 2 1 2 1 2 Slide 30 4 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 Slide 31 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 2,y 2 ; Line 3 x 1, y 1 ; x 2,y 2 ; x 3, y 3 ; x 4, y 4 ; Polygon Line 1, Line 2 Polygon Line 2, Line 3 (x1, y1) (x2, y2) 1 2 1 2 1 2 Line 1 Line 2 Line 3 Slide 32 4 Vector (2) Topological Structure Polygon = a list of lines Line = a set of coordinates Slide 33 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 Slide 34 Arc-Node Model Polygon topology table Node topology table Arc topology table Arc coordinate data table Slide 35 Slide 36 Readings Chapter 3