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 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., 40 0 F, 80 0 F Ratio - absolute quantities, e.g., , e, , …

Components 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 Objects and Field View Objects - Are discrete or have identifiable boundaries - Have attributes Field - Varies continuously - Has no obvious boundaries

Data Models Vector data model Raster data model courtesy: Mary Ruvane,

Data Models

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

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

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 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

Data Models - (2) Raster

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

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; 11,3; 3,1; 5,4 …

3 Raster Data Structures (2) Quadtrees Subdividing a region into quadrants until each quadrant contains only one class - variable resolution

3 (2) Quadtrees

4 Vector Data Structures (1) Non-topological Structure There is no topology and shared boundaries are recorded twice

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

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

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) Line 1 Line 2 Line 3

4 Vector (2) Topological Structure Polygon = a list of lines Line = a set of coordinates

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