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

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.

“Geographically Referenced”

Refers to data referenced by location.

• Latitude/Longitude• Northing/Easting• Standard Format• Many different conventions

The Components of a GISSeveral components constitute a GIS. The user becomes Several components constitute a GIS. The user becomes part of the GIS whenever complicated analyses, such a part of the GIS whenever complicated analyses, such a

spatial analysis and modeling, are carried out.spatial analysis and modeling, are carried out.

Using a GIS

Now more than ever, GIS is available to the Now more than ever, GIS is available to the health practitioner.health practitioner.

5 Years ago, extensive GIS analysis was limited to Unix 5 Years ago, extensive GIS analysis was limited to Unix computers and command line software packages.computers and command line software packages.

New user-friendly windows based software and Pentium New user-friendly windows based software and Pentium computer technology now places high-end GIS tools computer technology now places high-end GIS tools within your reach.within your reach.

A GIS Provides the

Ability to Analyze

Disparate Data Sets Based on

Location

Spatial Analysis:

Data 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

Data Linkage is Required to Conduct Spatial Queries

• Exact MatchingExact Matching– same set of features in same set of features in

both filesboth files• Hierarchical MatchingHierarchical Matching

– summing nested subsets summing nested subsets to get answerto get answer

• Fuzzy MatchingFuzzy Matching– boundaries do not matchboundaries do not match

Data can be linked in many ways.Data can be linked in many ways.

State County

Georgia Fulton

DeKalb

State County

Georgia Fulton

DeKalb

A GIS Can Perform these Operations

GIS uses geography, or space, as the common key GIS uses geography, or space, as the common key element between data sets. Information is linked element between data sets. Information is linked only if it relates to the same geographic area.only if it relates to the same geographic area.

•What is at…?What is at…?•Where is it...?Where is it...?•What has changed since…?What has changed since…?•What spatial patterns exist?What spatial patterns exist?•What if..?What if..?

Sample GIS Applications

GIS is being used around the world GIS is being used around the world for many purposes:for many purposes:

Environmental protection/restoration Environmental protection/restoration Natural resource managementNatural resource managementPower allocation by public utilitiesPower allocation by public utilitiesMarketing to identify target consumersMarketing to identify target consumersDisease outbreak surveillanceDisease outbreak surveillance

Consider the public health implications of the Consider the public health implications of the uses of GIS in all these important areas.uses of GIS in all these important areas.

Geographic Data Concepts

The principles behind spatial analysis.

GIS Attempts to Describe All Features in Geometric Terms.

• Point: discrete location• Line (Arcs): set of ordered coordinates• Polygon (Area): closed feature whose boundary

encloses a homogeneous area

Lines (Arcs)Points

1 32 4 2

31

Polygon (Area)

Distance Functions

Routing AreaAnalysis

GIS Attempts to Describe All Features in Geometric Terms.

• Points:Points: sampling sampling locations, disease cases, locations, disease cases, town centroidstown centroids

• Lines (Arcs):Lines (Arcs): streams, streams, power lines, power lines, transportation routestransportation routes

• Polygons (Areas):Polygons (Areas): land land use, lakes, census use, lakes, census tracts, tracts, town town boundariesboundaries

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.

What is Map?

• A map is a graphic representation of some A map is a graphic representation of some part of the earth’s surface.part of the earth’s surface.

• A map contains a series of themes or A map contains a series of themes or coverages that are often combined to form coverages that are often combined to form the final product.the final product.

• A map also contains descriptive A map also contains descriptive information which helps the reader information which helps the reader interpret the information on the map.interpret the information on the map.

Primary components of a map:

Map ImageMap Image

Trenton, NJ

Legend

Town

Water

Parks

Scale Bar

Map Scale

• Scale: describes the relation between a describes the relation between a single map unit to the number of same units single map unit to the number of same units in the real world.in the real world.

Example: 1:1000 (1 inch = 1000 inches)Example: 1:1000 (1 inch = 1000 inches)

• Scale Bar: compares the map units to an compares the map units to an established real-world unit of measure.established real-world unit of measure.

Example: 1 inch = 2.5 milesExample: 1 inch = 2.5 miles

This scale tells the user how the map relates to This scale tells the user how the map relates to the real world features it represents.the real world features it represents.

The Term ‘Map’ Also is Used to Describe a GIS Project or View

• A map is an interpretation of features on the earth’s 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.

Raster vs. Vector Data

Concept

Two methods exist for characterizing a location in space.

Raster-Based Analysis

• Area of analysis divided into squares of Area of analysis divided into squares of uniform size.uniform size.

• Each cell characterizes the feature of interest Each cell characterizes the feature of interest within this area with a single value.within this area with a single value.

Image Data Are Stored in Raster Format

GRID cell-based modeling uses the raster format to determine routing

patterns and terrain.

Aerial Photos & Satellite Imagery Stored in Raster Format

Vector Data

Computer algorithms are used to convert data of Computer algorithms are used to convert data of one type to the other.one type to the other.

• Coordinate-based data structure Coordinate-based data structure commonly used to represent commonly used to represent linear features.linear features.

• Each feature is represented as a Each feature is represented as a list of ordered x,y coordinateslist of ordered x,y coordinates

There are two basic types of information in GIS:

• Spatial informationSpatial information describes the location and shape of geographic features, and their spatial relationship to other features, and

• Descriptive informationDescriptive information which characterizes the geographic feature.

GIS Links Spatial Data with Geographic Information About a Particular Feature on a Map

The information is The information is stored as ‘attributes’ stored as ‘attributes’ of the graphically of the graphically represented feature.represented feature.

Example: A line that denotes a road tells you nothing but its location. Example: A line that denotes a road tells you nothing but its location. An An attribute tableattribute table stores all relevant information about this feature, stores all relevant information about this feature, which can be queried and displayed in a format based on the user’s which can be queried and displayed in a format based on the user’s needsneeds

Roads Map

1

4

32

5

6

Feature No. X,Y Pairs

1 3,5 5,52 5,5 8,53 8,5 9,54 6,9 5,4 5,7 5,6 5,55 5,5 4,4 4,16 0,5 3.2

Feature No.Road-Type Surface Width Lanes Name1 2 Asphalt 48 4 N. Main St.2 2 Asphalt 48 4 N. Main St.3 2 Asphalt 48 4 N. Main St.4 1 Concrete 60 4 Hwy. 425 1 Concrete 60 4 Hwy. 42

Feature List

Attribute Table

Coordinate Systems

• In a GIS, locations on the earth’s 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.

• Coordinate systems must be consistent between map Coordinate systems must be consistent between map layers.layers.

• For any database to be useful for spatial analysis, the For any database to be useful for spatial analysis, the database must be registered to a recognized global database must be registered to a recognized global coordinate system.coordinate system.

• A coordinate system consists of:A coordinate system consists of:

– A spheroid:A spheroid: a mathematical description of the a mathematical description of the earth’s shapeearth’s shape

– A map projection:A map projection: a mathematical conversion from a mathematical conversion from spherical to planar coordinatesspherical to planar coordinates

X & Y Coordinates Define the Location of Map Features

Map Projection

Map Projection

Map Projection

Resolution

• 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.

The accuracy with which a given map scale can The accuracy with which a given map scale can depict the location and shape of map featuresdepict the location and shape of map features

Resolution

Resolution Plays a Large Part in the Ability of a Map to Accurately Describe Earth’s 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.

What is Topology?

• 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).

Topology Is a Mathematical Procedure for Topology Is a Mathematical Procedure for Explicitly Defining Spatial Relationships. Explicitly Defining Spatial Relationships.

Connectivity

• Points along the arc that define its shape are called vertices.

• Endpoints of arcs are called nodes.

• Arcs join only at nodes.

Arc-Node TopologyArc-Node Topology

Area Definition

• Polygons are Polygons are represented as a represented as a series of x, y series of x, y coordinates that coordinates that connect to define an connect to define an area.area.

• The GIS also stores The GIS also stores the list of arcs that the list of arcs that make up the polygon.make up the polygon.

Polygon-Arc TopologyPolygon-Arc Topology

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.

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.

Data Sources

Electronic Data Files

This is the easiest way to get data into a GISThis 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

Digitizer

• Involves manually tracing all features of interest using an electronic stylus

• Good base maps must be used• Paper maps affected by climatic conditions

A digitizer converts spatial features on a hard copy map into digital format. Point, line and area features are converted into X, Y coordinates.

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.

Keyboard Entry

• Very accurate• Requires minimal conversion• Can be time intensive

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).

Product: Coverage

• Each theme should exist as a separate coverage• Different feature types can coexist in a coverage if

they describe the same data.

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:

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.

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.

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.

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.

The User Has the Responsibility of Doing the Research to Identify the Most Accurate and Relevant Spatial Data Set

• Don’t 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. Don’t rely on memory.