1. Spatial data models (types)
Lecture 3, 9/7/2006

2. Two basic data models to represent these features
Raster spatial data model
Define space as an array of equally sized cells arranged in rows and columns. Each cell contains an attribute value and location coordinates
Individual cells as building blocks for creating images of point, line, area, network and surface
Continuous raster
Numeric values range smoothly from one location to another, for example, DEM, temperature, remote sensing images, etc.
Discrete raster
Relative few possible values to repeat themselves in adjacent cells, for example, land use, soil types, etc.
Vector spatial data model
Use x-, y- coordinates to represent point, line, area, network, surface
Point as a single coordinate pair, line and polygon as ordered lists of vertices, while attributes are associated with each features
Usually are discrete features

3. DIGITAL SPATIAL DATA RASTER VECTOR Real World
This is an illustration of transferring a real world geographic area into the raster and vector formats. In the raster format the geographic area is parceled into numerous grid squares and the value that makes up the majority of the square dictates that that entire square gets the value. In the vector format all entities are classified into points, lines, or polygons.
Source: Defense Mapping School
National Imagery and Mapping Agency

4. Raster and Vector Data Models
Real World
600 1 2 3 4 5 6 7 8 9 10 1 B G
Trees
500 2 B G G 3 B
400 4 B G G
Trees
Y-AXIS 5 B G G
300 6 B
G G BK
House 7 B
200 8 B B
River 9 B
100 10 B
100
200
300
400
500
600
X-AXIS
Raster Representation
Vector Representation
Source: Defense Mapping School
National Imagery and Mapping Agency

5. Example: Discrete raster

6. Example: continuous raster
Xie et al. 2005

7. Raster
Real world
Vector
Heywood et al. 2006

8. Effects of changing resolution
Heywood et al. 2006

9. Vector – Advantages and Disadvantages
Good representation of reality
Compact data structure
Topology can be described in a network
Accurate graphics
Disadvantages
Complex data structures
Simulation may be difficult
Some spatial analysis is difficult or impossible to perform

10. Raster – Advantages and Disadvantages
Simple data structure
Easy overlay
Various kinds of spatial analysis
Uniform size and shape
Cheaper technology
Disadvantages
Large amount of data
Less “pretty”
Projection transformation is difficult
Different scales between layers can be a nightmare
May lose information due to generalization

11. GIS data formats (files)
Shapefiles
Coverages
TIN (e.g. elevation can be stored as TIN)
Triangulated Irregular Network
Grid (e.g. elevation can be stored as Grid)
Image (e.g. elevation can be stored as image)
Vector data
Raster data

12. Shape Files Nontopological Advantages no overhead to process topology
Disadvantages polygons are double digitized, no topologic data checking
At least 3 files .shp .shx .dbf

13. Coverages Original ArcInfo Format Directory With Several Files
Database Files are stored in the Info Directory
Uses Arc Node Topology
Containment (coincident)
Connectivity
Adjacency

14. TIN ©Arthur J. Lembo Cornell University
A triangulated irregular network (TIN) is a data model that is used to represent three dimensional objects. In this case, x,y, and z values represent points. Using methods of computational geometry, the points are connected into what is called a triangulation, forming a network of triangles. The lines of the triangles are called edges, and the interior area is called a face, or facet.
While the TIN model is somewhat more complex than the simple point, line, and polygon vector model, or the raster model, it is actually quite useful for representing elevations. For example a raster grid would require grid cells to cover the entire surface of a geographic area. Also, if we wanted to show great detail we would have to have small grid cells. Now, if the land area is relatively flat, we would still need the small grid cells. However, with a TIN we would not have to include so many points on the flat areas, but could add more points on the steep areas where we want to show greater detail.
The illustration shows how we can create a TIN of the terrain around Ithaca, NY.
First, a series of elevation points are created
Second, a TIN face is created with the elevation data
Third, the faces are shaded in to give the impression of a 3D surface

15. Components of a TIN Nodes Edges Triangles Hull Topology
©Arthur J. Lembo
Cornell University

16. Grid Properties Each Grid Cell holds one value even if it is empty.
A cell can hold an index standing for an attribute.
Cell resolution is given as its size on the ground.
Point and Lines move to the center of the cell.
Minimum line width is one cell.
Rasters are easy to read and write, and easy to draw on the screen.