# Presentation: 262514

## 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.

## 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

***Exact Matching***
***same set of features in both files***
***Hierarchical Matching***
***summing nested subsets to get answer***
***Fuzzy Matching***
***boundaries do not match***

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

## A GIS Can Perform these Operations

**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.**

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

## 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.

- Point: discrete location
- Line (Arcs):  set of ordered coordinates
- Polygon (Area):  closed feature whose boundary encloses a homogeneous area

**Lines (Arcs)**

**Points**

**1**

**3**

**2**

**4**

**2**

**3**

**1**

**Polygon (Area)**

**Distance **
**Functions**

**Routing**

**Area**
**Analysis**

## 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:

- Map Image

**Trenton, NJ**

- _Legend_

- Town

- Water

- Parks

- Scale Bar

## Map Scale

***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**

**This scale tells the user how the map relates to 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 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

## Vector Data

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

**Coordinate-based data structure commonly used to represent linear features.**
**Each feature is represented as a list of ordered x,y coordinates**

## 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.

**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**

- Roads Map

**1**

**4**

**3**

**2**

**5**

**6**

- 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 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

**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**

- 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 depict 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 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 Topology**

## Area Definition

**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.**

**Polygon-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 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.