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Introduction to Geographic Information Systems for Public Health Protection

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Title: Introduction to Geographic Information Systems for Public Health Protection


1
Introduction to Geographic Information Systems
for Public Health Protection
  • William D. Henriques, Ph.D., MSPH
  • Environmental Toxicologist, GIS Coordinator
  • Agency for Toxic Substances and Disease Registry
  • Atlanta, Georgia

2
What is a GIS?
A GIS is 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.

3
Geographically Referenced
  • Refers to data referenced by location.
  • Latitude/Longitude
  • Northing/Easting
  • Standard Format
  • Many different conventions

4
The Components of a GIS
Several components constitute a GIS. The user
becomes part of the GIS whenever complicated
analyses, such a spatial analysis and modeling,
are carried out.
5
Using a GIS
Now more than ever, GIS is available to the
health practitioner. 5 Years ago, extensive GIS
analysis was limited to Unix computers and
command line software packages. New user-friendly
windows based software and Pentium computer
technology now places high-end GIS tools within
your reach.
6
A GIS Provides the Ability to Analyze Disparate
Data Sets Based on Location
7
Spatial AnalysisData queries on georeferenced
information.
  • How many people live within one mile of a
    Superfund hazardous waste site in the United
    States?
  • Analysis requires data linkage
  • Sometimes in the same data set
  • Sometimes in a second data set

8
Data Linkage is Required to Conduct Spatial
Queries
Data can be linked in many ways.
  • Exact Matching
  • same set of features in both files
  • Hierarchical Matching
  • summing nested subsets to get answer
  • Fuzzy Matching
  • boundaries do not match

9
A GIS Can Perform these Operations
  • What is at?
  • Where is it...?
  • What has changed since?
  • What spatial patterns exist?
  • What if..?

GIS uses geography, or space, as the common key
element between data sets. Information is linked
only if it relates to the same geographic area.
10
Sample GIS Applications
GIS is being used around the world for many
purposes
  • Environmental protection/restoration
  • Natural resource management
  • Power allocation by public utilities
  • Marketing to identify target consumers
  • Disease outbreak surveillance

Consider the public health implications of the
uses of GIS in all these important areas.
11
Geographic Data Concepts
  • The principles behind spatial analysis.

12
GIS Attempts to Describe All Features in
Geometric Terms.
Polygon (Area)
Lines (Arcs)
Points
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Routing
Distance Functions
Area Analysis
  • Point discrete location
  • Line (Arcs) set of ordered coordinates
  • Polygon (Area) closed feature whose boundary
    encloses a homogeneous area

13
GIS Attempts to Describe All Features in
Geometric Terms.
  • Points sampling locations, disease cases, town
    centroids
  • Lines (Arcs) streams, power lines,
    transportation routes
  • Polygons (Areas) land use, lakes, census
    tracts, town boundaries

14
Concepts to Remember
  • Many features can be described by either a point
    or a polygon.
  • Similarly, lines can be of a specific width.
  • Map scale and resolution define the conditions
    for appropriate application of these feature
    types.
  • The uses of coordinate based analysis is only
    limited by the imagination of the user.

15
What is Map?
  • A map is a graphic representation of some part of
    the earths surface.
  • A map contains a series of themes or coverages
    that are often combined to form the final
    product.
  • A map also contains descriptive information which
    helps the reader interpret the information on the
    map.

16
Primary components of a map
Trenton, NJ
Legend
Map Image
Town
Parks
Water
Scale Bar
17
Map Scale
This scale tells the user how the map relates to
the real world features it represents.
  • Scale describes the relation between a single
    map unit to the number of same units in the real
    world.
  • Example 11000 (1 inch 1000 inches)
  • Scale Bar compares the map units to an
    established real-world unit of measure.
  • Example 1 inch 2.5 miles

18
The Term Map Also is Used to Describe a GIS
Project or View
  • A map is an interpretation of features on the
    earths surface
  • Scale, map units, data layers (themes,
    coverages), are inherently part of a GIS
  • These functions are available to conduct spatial
    queries and measure distance in your project when
    you need them.

19
Raster vs. Vector Data Concept
  • Two methods exist for characterizing a location
    in space.

20
Raster-Based Analysis
  • Area of analysis divided into squares of uniform
    size.
  • Each cell characterizes the feature of interest
    within this area with a single value.

21
Image Data Are Stored in Raster Format
  • GRID cell-based modeling uses the raster format
    to determine routing patterns and terrain.

22
Aerial Photos Satellite Imagery Stored in
Raster Format
23
Vector Data
  • Coordinate-based data structure commonly used to
    represent linear features.
  • Each feature is represented as a list of ordered
    x,y coordinates

Computer algorithms are used to convert data of
one type to the other.
24
There are two basic types of information in GIS
  • Spatial information describes the location and
    shape of geographic features, and their spatial
    relationship to other features, and
  • Descriptive information which characterizes the
    geographic feature.

25
GIS Links Spatial Data with Geographic
Information About a Particular Feature on a Map
The information is stored as attributes of the
graphically represented feature.
Feature List
Roads Map
Attribute Table
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Example A line that denotes a road tells you
nothing but its location. An attribute table
stores all relevant information about this
feature, which can be queried and displayed in a
format based on the users needs
26
Coordinate Systems
  • In a GIS, locations on the earths surface
    described by points, lines, and polygons are
    defined by a series of X, Y coordinates
  • Coordinate systems can be self-described or in
    units that relate to the real world.
  • Decimal degrees degrees, minutes, seconds
    meters and feet are all examples of units of
    measure in a coordinate system.

27
X Y Coordinates Define the Location of Map
Features
  • Coordinate systems must be consistent between map
    layers.
  • For any database to be useful for spatial
    analysis, the database must be registered to a
    recognized global coordinate system.
  • A coordinate system consists of
  • A spheroid a mathematical description of the
    earths shape
  • A map projection a mathematical conversion from
    spherical to planar coordinates

28
Map Projection
29
Map Projection
30
Map Projection
31
Resolution
The accuracy with which a given map scale can
depict the location and shape of map features
  • The larger the map scale, the higher the possible
    resolution.
  • As map scale decreases, resolution diminishes and
    feature boundaries must be smoothed, simplified,
    or not shown at all.
  • Resolution plays a large role in GIS, especially
    in raster-based modeling.

32
Resolution
33
Resolution Plays a Large Part in the Ability of a
Map to Accurately Describe Earths Features
  • Essential that the user be mindful of the scale
    of the data layers
  • Serious errors can result if the theme lacks
    sufficient resolution to effectively describe an
    area of interest.
  • A GIS does not tell you that you have made an
    error in choosing the right data layer for your
    project.

34
What is Topology?
Topology Is a Mathematical Procedure for
Explicitly Defining Spatial Relationships.
  • Arcs connect to each other at nodes
    (connectivity),
  • Arcs that connect to surround an area define a
    polygon (area definition), and
  • Arcs have direction and right and left sides
    (contiguity).

35
Connectivity
Arc-Node Topology
  • Points along the arc that define its shape are
    called vertices.
  • Endpoints of arcs are called nodes.
  • Arcs join only at nodes.

36
Area Definition
Polygon-Arc Topology
  • Polygons are represented as a series of x, y
    coordinates that connect to define an area.
  • The GIS also stores the list of arcs that make up
    the polygon.

37
Contiguity
  • Every arc has a direction.
  • The GIS maintains a list of polygons on the left
    and right side of each arc.
  • The computer uses this information to determine
    which features are next to one another.

38
Getting Data into a GIS
  • Digitizing hard copy maps,
  • Keyboard entry of coordinate data,
  • Electronic entry using a data file,
  • Scanning a map manuscript, and
  • Converting or reformatting existing data.

39
Data Sources
40
Electronic Data Files
This is the easiest way to get data into a GIS
  • Ready-to-use data sources include
  • DXF Auto-CAD files/Scanning products
  • DLG Digital Line Graphs available from the USGS
  • TIGER 1990 Census files
  • SHP ArcView Shape files
  • Can also add point data using dBase file with X,
    Y coordinates in decimal degrees

41
Digitizer
A digitizer converts spatial features on a hard
copy map into digital format. Point, line and
area features are converted into X, Y coordinates.
  • Involves manually tracing all features of
    interest using an electronic stylus
  • Good base maps must be used
  • Paper maps affected by climatic conditions

42
Digitizer
  • After digitizing, a procedure known as
    transformation converts digitizer units to a
    real-world coordinate system.
  • Tics are used to provide the relationship between
    the two coordinate systems.

43
Keyboard Entry
Coordinates are added as a series of numbers
defining the location of a point, the shape of a
line, or the coordinates that define a closed
area (polygon).
  • Very accurate
  • Requires minimal conversion
  • Can be time intensive

44
Product Coverage
This term is used in a GIS to describe a spatial
data set that has a particular theme. A
coverage consists of topologically linked
geographic features. For maximal analytical power
  • Each theme should exist as a separate coverage
  • Different feature types can coexist in a coverage
    if they describe the same data.

45
Spatial Data Sources
  • Now more than ever, ready-made spatial data
    sources are available to quickly start GIS-based
    analyses.
  • Federal agencies like the Census bureau and the
    US Geological Service provide nationwide (and
    worldwide) spatial data sources.
  • Research these data sources to determine their
    applicability to your project.

46
Spatial Data Sources
  • Commercial vendors (e.g., Wessex, GDT) have
    converted some of these government products into
    formats that can be quickly used in the most
    popular GIS software.

47
Many agencies at the Federal and State level have
data available on the Internet
  • Web sites exist that provide pointers to some of
    the most useful spatial data sources.

48
The principles of GIS are based on data sharing.
No one group can (or should) do it alone.
  • An Executive Order requires Federal agencies to
    provide descriptions (metadata) of their data,
    and distribute it via the Internet.
  • Spatial data quality standards are now in place
    to help users understand what is out there and
    the intended purpose of the data set.

49
The User Has the Responsibility of Doing the
Research to Identify the Most Accurate and
Relevant Spatial Data Set
  • Dont settle for only one source contact other
    GIS professionals and obtain the data that fits
    your needs and meets the resolution demands of
    your project.
  • Document the source and other relevant
    information regarding the data set if others have
    failed to. Dont rely on memory.
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