Spatial data models (types)
Lecture 3, 9/7/2006
Two basic data models to represent these
features
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Raster spatial data model
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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
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Discrete raster
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Numeric values range smoothly from one location to another, for example,
DEM, temperature, remote sensing images, etc.
Relative few possible values to repeat themselves in adjacent cells, for
example, land use, soil types, etc.
Vector spatial data model
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Use x-, y- coordinates to represent point, line, area, network, surface
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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
DIGITAL SPATIAL DATA
• RASTER
• VECTOR
• Real World
Source: Defense Mapping School
National Imagery and Mapping Agency
Raster and Vector Data Models
Real World
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Raster Representation
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Trees
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Y-AXIS
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Trees
House
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River
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X-AXIS
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Vector Representation
Source: Defense Mapping School
National Imagery and Mapping Agency
Example: Discrete raster
Example: continuous raster
Xie et al. 2005
Raster
Real world
Vector
Heywood et al. 2006
Effects of changing resolution
Heywood et al. 2006
Vector – Advantages and
Disadvantages
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Advantages
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Good representation of reality
Compact data structure
Topology can be described in a network
Accurate graphics
Disadvantages
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Complex data structures
Simulation may be difficult
Some spatial analysis is difficult or impossible to perform
Raster – Advantages and
Disadvantages
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Advantages
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Simple data structure
Easy overlay
Various kinds of spatial analysis
Uniform size and shape
Cheaper technology
Disadvantages
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Large amount of data
Less “pretty”
Projection transformation is difficult
Different scales between layers can be a nightmare
May lose information due to generalization
GIS data formats (files)
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Vector data
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Shapefiles
Coverages
TIN (e.g. elevation can be stored as TIN)
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Raster data
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Triangulated Irregular Network
Grid (e.g. elevation can be stored as Grid)
Image (e.g. elevation can be stored as image)
Shape Files
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Nontopological
Advantages no overhead to process topology
Disadvantages polygons are double
digitized, no topologic data checking
At least 3 files .shp .shx .dbf
Coverages
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Original ArcInfo Format
Directory With Several Files
Database Files are stored in the Info
Directory
Uses Arc Node Topology
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Containment (coincident)
Connectivity
Adjacency
©Arthur J. Lembo
Cornell University
TIN
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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.
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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
Components of a TIN
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Nodes
Edges
Triangles
Hull
Topology
©Arthur J. Lembo
Cornell University
Grid Properties
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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.