Title: Introduction to Geographic Information Systems for Public Health Protection
1Introduction 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
2What 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.
3Geographically Referenced
- Refers to data referenced by location.
- Latitude/Longitude
- Northing/Easting
- Standard Format
- Many different conventions
4The 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.
5Using 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.
6A GIS Provides the Ability to Analyze Disparate
Data Sets Based on Location
7Spatial 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
8Data 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
9A 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.
10Sample 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.
11Geographic Data Concepts
- The principles behind spatial analysis.
12GIS 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
13GIS 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
14Concepts 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.
15What 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.
16Primary components of a map
Trenton, NJ
Legend
Map Image
Town
Parks
Water
Scale Bar
17Map 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
18The 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.
19Raster vs. Vector Data Concept
- Two methods exist for characterizing a location
in space.
20Raster-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.
21Image Data Are Stored in Raster Format
- GRID cell-based modeling uses the raster format
to determine routing patterns and terrain.
22Aerial Photos Satellite Imagery Stored in
Raster Format
23Vector 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.
24There 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.
25GIS 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
26Coordinate 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.
27X 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
28Map Projection
29Map Projection
30Map Projection
31Resolution
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.
32Resolution
33Resolution 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.
34What 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).
35Connectivity
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.
36Area 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.
37Contiguity
- 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.
38Getting 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.
39Data Sources
40Electronic 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 -
41Digitizer
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
42Digitizer
- 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.
43Keyboard 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
44Product 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.
45Spatial 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.
46Spatial 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.
47Many 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.
48The 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.
49The 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.