Introduction to GIS

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Introduction to GIS
July 5, 2006

• Instructor
• Sky Harrison
• USGS Pacific Basin Information Node
• http\\pbin.nbii.gov

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Definition of GIS (1)

• Geographic Information System
• An organized collection of computer hardware,
  software, geographic data, and personnel designed
  to efficiently capture, store, update,
  manipulate, analyze and display all forms of
  geographically referenced information.
• from Understanding GISThe ARC/INFO Method,
  ESRI, 1993

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Definition of GIS (2)

• Geographic Information System
• A set of tools for collecting, storing,
  retrieving at will, transforming, and displaying
  spatial data from the real world for a particular
  set of purposes.
• from Principles of Geographical Information
  Systems for Land Resources Assessment, P.A.
  Burrough, 1986.

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GIS Components
People
GIS
Data
Software
Analysis
Hardware
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What can GIS do?

• Data storage and management
• Based on a data base that contains both spatial
  information (where is a feature located) and
  attribute information (what is the feature
• Spatial Analysis
• Display of results
• Making maps

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Spatial Analysis

• Questions a GIS can answer
• 1. What is at...? (location)
• 2. Where is it? (condition)
• 3. What has changed since...? (trends)
• 4. What spatial patterns exist? (patterns)
• 5. What if...? (modeling)

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What do we think of when we think of a Map?

• Maps often represent features as point, lines or
  polygons (vector Model)
• How a feature is represented is scale dependent

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Vector Data Model

• Basic Element the point (x,y)
• Points connect to make lines.
• Lines connect to make polygons.

(x2,y2)
(x2,y2)
(x1,y1)
(x,y)
(x3,y3)
(x1,y1)
Point
Line
Polygons
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Vector Data Model

• Best for representing discrete objects, e.g.
• observation locations
• roads
• legal boundaries
• Most paper maps use the vector data model

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Raster Data Model

• Basic element the grid cell
• Geographic features are represented on a grid of
  square cells, all the same size.
• What the computer does
• remembers line, column, value
• numeric attribute values
• can calculate all x,y coordinates

Columns -----gt
Lines -------gt
(x,y)
Known grid cell size, e.g. 20 m
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Digital Elevation Models Are An Example of a
Raster Dataset
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Vector vs. Raster
Raster is faster, but Vector is correcter

• Raster is computationally efficient, because
  computers are good at matricies and because
  everything lies neatly on top of each other,
• but Raster generalizes spatial location.
• Vector is more spatially accurate, because it can
  place points anywhere,
• but Vector is slower, because the calculations
  are much more intensive.

In practice, which data model depends on software
and data.
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What Is a Data Layer?

• Two components
• Spatial data - where something is
• Attribute data - describe the something

Special Status Species Observations
ID Date Species 11 010999 Tiger 12 030999 Guar 14
040999 Samba Deer ...
Spatial and attribute data are linked
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Attribute Data

• Tables are defined by fields
• Field definitions include the data type (e.g.
  number, string, boolean, date) and the width of
  the field

Fields
ID Date Species 11 010699 Tiger 12 030999 Guar 14
040999 Samba Deer ...
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GIS Data Layers

• GIS Themes, or Layers

Vegetation
Land Ownership
Roads
Rivers
Special Status Species Locations
All Layers
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Scale and GIS

• GIS can use any scale datascale is dependent on
  the data.
• GIS is scale independent.
• Questions and answers are scale dependent.
• Applications range from the back of a leaf to
  entire planets.

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Scale

• Ratio between distances on the map and
  corresponding distances in the real world
• (i.e. scale of 1100,000 means 1cm on map
  represents 100,000 cm, or 1 km on the Earths
  surface)
• Small scale vs. Large scale
• large scale shows greater detail
• small scale has larger number in representative
  fraction
• Scale controls how features are shown and what
  features are shown
• (i.e. large scale, cities are polys small scale,
  they are points)

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Compatibility Issues

• In GIS, you must use data of with a scale that is
  appropriate for the application
• for city planning, you would NOT use 11million
  scale data
• for global modeling, you would NOT use 12,000
  scale data
• Data of same or similar scale should be used for
  an application
• Do not combine data layers of widely diverse
  scales
• If different scale data are combined, analysis
  and conclusions may be flawed

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How do we make a round world flat?

• Answer
• Map Projections

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Map Projections

• Projection is a method by which the curved
  surface of the earth is represented on a flat
  surface
• Distortions are inevitable
• Mathematical transformations are used

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Map Projections

• Lead to four kinds of distortion
• area
• distance
• direction
• shape
• Projections can be optimized to minimize one kind
  of distortion
• equal area preserves area
• equal distance preserves distance
• true direction preserves direction
• conformal preserves shape

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Map Projections and Coordinate Systems

• Coordinate systems determine what the x,y
  coordinates mean.
• The coordinate system depends on the map
  projection, the datum and the spheroid.
• Projection projecting 3D information into 2D
• Spheroid defines the shape of the Earth
• Datum defines the origin and the reference
  directions

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Latitude and Longitude

• A Spherical (not Planar) Coordinate System
• Latitude angle traced between the Equator, the
  center of the Earth and the point.
• Longitude angle traced between the Greenwich
  Meridian, the center of the Earth and the point.
• Angle expressed in Degree-Minute-Seconds (DMS) or
  in Decimal Degrees (DD)

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Latitude and Longitude
Longitude Latitude
- Longitude Latitude
New York City 40.40 N 73.58 W
North Latitude
Lhasa 29 24 36 N 91 7 12 W
East Longitude
West Longitude
Equator Latitude 0
Greenwich Meridian Longitude 0
South Latitude
Longitude - Latitude
- Longitude - Latitude
Buenos Aires -34.20 latitude -58.30 longitude
Latitude and Longitude can be expressed in
several ways.
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Multiple Data Layers

• For data layers to be used together, they must
  be
• in the same coordinate system
• at comparable scale (similar grain and extent)
• and for some analysis, in the same data model
  (vector or raster)

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How Do You Know Your Scale and Projection?

• Metadata

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What Are Metadata?

• "Data about data"
• Metadata describe the content, quality,
  condition, and other characteristics of data.

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Metadata and Data Dictionaries

• Standards for Digital Geospatial Metadata
• Federal Geographic Data Committee
• 53 pages of standards about metadata
• Data Dictionary
• description of each field of attributes
• meaning of codes and abbreviations
• data field definitions

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Metadata

• Major uses of metadata
• organize and maintain an organization's
  investment in data.
• provide information to data catalogs and
  clearinghouses.
• provide information to aid data transfer.

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Software Choices

• ArcGIS (formerly ArcInfo and ArcView)
• Environmental Systems Research Institute
• industry leader
• Intergraph
• Erdas
• GRASS
• MapInfo
• Atlas GIS (now owned by ESRI)
• IDRISI
• Many others

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Data Sources

• Methods for transferring data into a GIS
• Maps (on paper)
• Remote Sensing and Aerial Photography
• Graphic-file images
• Global Position Systems (GPS)
• Sources of available GIS data

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Sources of Data

• Hawaii Statewide GIS Program
• http//www.hawaii.gov/dbedt/gis/
• World Wide Web
• Data Clearinghouses
• Public agencies
• Networking!