GIS Data Models

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GIS Data Models
Lecture 2-3, Jan. 26 and 28, 2004

• Vector Data Models
• Vector File Formats
• Raster Data Models
• Raster File Formats

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Data Abstraction

• To use GIS the real world must be abstracted into
  points, lines, polygons, raster cells, and
  attribute values
• Class examples may use common object that most
  people will understand. If you understand how to
  abstract common objects you will be able to apply
  the same method to object in your field

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What is Vector Data

• Vector Data uses Points and their (X,Y)
  coordinates to represent spatial features
• Points, Lines and Polygons

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Points

• A point is a 0 dimensional object and has only
  the property of location (x,y)
• Points can be used to Model features such as a
  well, building, power, pole, sample location ect.
• Other name for a point are vertex, node, 0-cell

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Lines

• A line is a one-dimensional object that has the
  property of length
• Lines can be used to represent road, streams,
  faults, dikes, maker beds, boundary, contacts
  etc.
• Lines are also called an edge, link, chain, arc,
  1-cell
• In an ArcInfo coverage an arc starts with a node,
  has zero or more vertices, and ends with a node

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Figure 4.3 p58 Bernhardsen
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Polygons

• A polygon is a two-dimensional object with
  properties of area and perimeter
• A polygon can represent a city, geologic
  formation, dike, lake, river, ect.
• Other name for polygons face, zone 2-cell
• Scale matters

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Data Abstraction Discussion

• If you do not understand this the rest of GIS
  will not make sense
• Scale Matters
• Intended use Matters

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Examples

• Trees
• Dikes
• Roads
• Rivers

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Topology

• A set of rules on how objects relate to each
  other
• Major difference in file formats
• Higher level objects have special topology rules

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Topology Definition

• The Science of mathematics of relationships used
  to validate the geometry of vector entities, and
  for operations such as network tracing and tests
  of polygon adjacency.
• The study of geometric properties that do not
  change when the forms are bent, stretched or
  under go similar geometric transformations.

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Figure 2-9 GIS Fundamentals, Bolstad
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Planer Enforcement
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Why Topology Matters

• Error Detection
• open polygons
• unlabeled polygons
• slivers
• polygons that cannot exist next to each
• other
• Network Modeling

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Show Placitas

• Arc Node Topology
• Cover
• Lpoly and Rpoly
• Tnode fnode
• Label errors

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Higher Level Object

• Regions
• Networks
• TIN Triangulated irregular network
• Dynamic Segmentation

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Regions

• Overlapping areas with different attributes
• Fire history
• Disconnected areas with the same attributes
• Hawaii

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Networks

• Road systems, power grids, water supply sewerage
  systems, drainage network
• Continuous connected networks
• Rules for displacement in a network
• Attribute value accumulations due to
  displacements

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TIN

• Vector Surface Model
• Triangulated Irregular Network
• A set of nonoverlapping triangles each with a
  constant gradient
• A TIN can honor original input elevations

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Dynamic Segmentation

• Combines a line coverage with a linear reference
  system
• Has event tables for point events and linear
  events

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Fig 3.13 p52 Chang
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Examples and Demo
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Shape Files

• Nontopological
• Advantages no overhead to process topology
• Disadvantages polygons are double digitized, no
  topologic data checking
• 3 files .shp .shx .dbf

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Coverages

• Original ArcInfo Format
• Directory With Several Files
• Database Files are stored in the Info Directory
• Uses Arc Node Topology
• Planer Enforcement
• Connectivity
• Adjacency

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GeoDatabase

• New GIS Format at ArcGIS 8.0
• Two Types
• Personal Geodatabase
• Microsoft access 2000 database
• SDE GeoDatabase
• Multi-user
• Can connect to many RDBMS
• Oracle, SQL server, Informix
• File are stored in the format native to the RDBMS

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Box 3.5 Geographic Information Systems, Chang 04
p. 55
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GeoDatabase

• Shapes are similar to shape files
• Object-oriented model not a Geo-relational
• There are 25 topology rules than can be used to
  relate different layers

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Raster Data Model
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Figure 31 Getting Started with Geographic
Information Systems, Clarke (2003) p. 91
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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.

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Raster Pyramids

• With out pyramids the entire raster must be read
  for each screen draw
• Pyramids store reduced resolution dataset files
  .rrd to increase the speed of screen draws
• When you add a raster to ArcMap if pyramids do
  not exist you can create them

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Raster Resampling

• Nearest Neighbor
• Closest cell
• Continuous and Discrete data
• Bilinear interpolation
• Average of nearest 4 cells
• Continuous data only
• Cubic Convolution
• Average of nearest 16 cells
• Continuous data only

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Quad Tree Compression

• May be use to get variable resolution for imagery
  in the National Map

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Figure 4.35 Geographic Information Systems and
Introduction, Bernhardsen (2001), p. 87
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C. Dana Tomlin, Geographic Information Systems
and Cartographic Modeling (1990), P. 44

• Yes raster is faster, but raster is vaster, and
  vector just seems more corrector

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Images are a form of raster data

• ArcGIS can use many common image formats

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Industry Standard Data Models

• Some Industries have created standard data models
• It is a good idea to use a standard model to
  promote sharing of data
• Some data models can be very complex
• Complex models require custom tools to be useful

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References

• Getting Started with Geographic Information
  Systems 4th Edition, Clark (2003)
• Geographic Information Systems an Introduction
  3rd Edition, Bernhardsen (2002)
• Introduction to Geographic Information Systems
  2nd Edition, Chang (2004)
• GIS Fundamentals, Bolstad (2002)
• ArcGIS 8.3 Desktop Help
• Using GRID with ArcInfo version 7 ESRI