Agency for Toxic Substances and Disease Registry 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 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.

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

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 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

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.

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.

Geographic Data Concepts The principles behind spatial analysis.

GIS Attempts to Describe All Features in Geometric Terms. Points Lines (Arcs) Polygon (Area) 1 1 3 3 2 4 2 Distance Functions Routing Area Analysis Point: discrete location Line (Arcs): set of ordered coordinates Polygon (Area): closed feature whose boundary encloses a homogeneous area

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

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 part of the earth’s 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.

Primary components of a map: Trenton, NJ Map Image Legend Town Parks Water Scale Bar

Example: 1:1000 (1 inch = 1000 inches) 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: 1:1000 (1 inch = 1000 inches) Scale Bar: compares the map units to an established real-world unit of measure. Example: 1 inch = 2.5 miles

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 uniform size. Each cell characterizes the feature of interest 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

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

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.

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 4 6 1 2 3 5 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 user’s needs

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.

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 earth’s shape A map projection: a mathematical conversion from spherical to planar coordinates

Map Projection

Map Projection

Map Projection

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.

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), 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).

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.

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.

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 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 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

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 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

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.

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 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.