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What is GIS?

I. What is GIS?

II. Mapping GIS data

What is GIS?

A Geographic Information System (GIS) is a computer system that stores, manages, maps and analyzes geographic data. Geographic data combines and links graphic representations of "features", or "events" found on the earth with corresponding tabular data, which is also called attribute data. The following example illustrates this concept with schools. Schools are represented here by single point locations. In a single record of information, each point has associated with it not only its exact location on the earth but also information about the school, such as its name, and the type of school it is.

GIS School Example

Features or events can be things as diverse as lightpoles, rivers, accident locations, delivery routes, historic battlegrounds, or the distribution of malaria cases. The linking of geographic information, with more traditional types of information is what makes a GIS unique and is the basis for all GIS functionality.

What does GIS data look like?

GIS data is comprised of graphical features (lines, points and polygons for example) and a table associated with each feature. The graphical features show a location on a map. Each individual feature has a record associated with it. Using query to pick out particular geographic information based on a set of rules would be considered geographic analysis. This example illustrates this concept.

Each of the school points on the map has information attached to it.

Example of GIS Data

Take the house location and calculate a five-mile radius around it.

GIS Calculations

Then the system is queried to find all certain schools and locate them.

Example of GIS Query

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How is GIS data stored and used?

A GIS stores information about the features and events in a collection of thematic "layers" or coverages as the following graphic illustrates.

Info as Thematic "Layers"

In fact, it is the most distinguishing feature of a GIS to find and display (map) the location of features or events. For example, a traditional computer system may store all information about schools. It will provide the user with a list of all the schools, possibly including addresses, but the user still doesn't know where exactly the schools are. The answer to such a simple questions as: "Are the schools evenly distributed across town?" is impossible to give with traditional computer systems. The following example will illustrate this simple functionality. It will show how locational information alone, without any further analysis, is very useful.

A maintenance crew for lightpoles is scheduled to replace all lightpoles in town that are over 50 years old. The crew has a database about the lightpoles that includes the date of installation. In a traditional database, they could query the database and get a list of the lightpoles that are over 50 years old. But, where are they? The GIS combines the location and the age information and immediately displays a map that shows the location of all lightpoles that are over 50 years old. It is much easier now to schedule work for the crew in an efficient manner, because they can quickly develop a route through town to replace the lightpoles.

Lightpoles and other physical features such as a road, are also called features with an "explicit" geographic location. This means they have a position on the earth that can be measured and described, usually in longitude/latitude, or values from an established grid system. To actually locate an address, or other "implicit" geographic references (i.e. those that don't have a permanent physical location) on a map, a process called geocoding is used to assign a specific location code in a longitude/latitude, or other gridsystem value. They then are linked with an explicit geographic reference. Once all addresses in a GIS are geocoded, all databases that include addresses can be linked and the addresses automatically located.

The functionality of a GIS can be applied to a wide variety of business sectors. It can be used to manage a telecommunications system, or forest resources, it can track the number of voters in a voting precinct and insure equal distribution of voters by re-drawing boundaries when necessary, and it can help determine the best location of a new business based on factors such as income statistics, competitor locations and ease of access. The options are endless and are continually evolving.

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What can a GIS do?

A GIS offers a wide variety of tools to manipulate, query, analyze and visualize geographic data. The type of analytical tools that GIS offers are often referred to as geographic, or spatial analysis. The location of a feature or event is what drives the analysis. Before GIS was available, geographic analysis was very cumbersome, sometimes impossible, and often simply not done. However, in today's environment, critical questions are often asked that require this very capability. For example:

"What is the fastest route to the emergency?"

"Which commercial district in town generated the most sales tax revenues?"

"Which properties in town are between 20 and 30 acres, with zoning that allows for a recycling facility?"

"Which homes are within 500 feet of the floodplain?"

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Types of GIS Analyses

A GIS offers the ability to find answers to these questions quickly. There are too many functions to attempt to illustrate all of them, but there are two types that are used often in local government that we'd like to explain here.

1) The first one is called "Proximity Analysis". The type of question that is answered with this type of analysis is: "Which homes or how many people are within a certain distance of this development proposal/railroad/bus route?" In the following hypothetical example, the City has received a request for a liquor license. City Code requires that there be no schools within a five-mile radius of a business that requires a liquor license. All schools that are located within a 5-mile radius of the building for which the liquor license was requested need to be identified.

Ex. of Proximity Analysis


2) Another type of analysis that is used frequently is called "Overlay analysis." The term "overlay" refers to the fact that conceptually different "layers" of information are put on top of each other to find geographically coinciding areas that match the required criteria. For example, this is used to answer the above question: "Which properties in town are between 20 and 30 acres, with zoning that allows for a recycling facility?" The following example illustrates this concept.

(Sample not yet developed!)

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II.  Mapping GIS data

What Makes a Good Map?

A good map is a map that clearly communicates its message to the reader. It should be easy to interpret map elements based on the symbology and graphics used, get oriented, and determine things such as distances and directions. Obviously, good cartography is critical; however, professionally produced maps typically include several elements outside of the actual mapped area, such as a scale bar, a legend and a north arrow, and for good reason. A scale indicates the relationship between a defined distance on the ground and the corresponding measurement on the map. Adding a scale bar to a map will make all measurements much easier. A north arrow aids in easy orientation. Without that, it would be often impossible to judge directions, for example where the southern exposure of a building is. A legend or a key is a guide to how features on the earth are symbolized on the map. For example, streets are often symbolized by lines of varying thickness. Including these elements in your maps will increase the effectiveness and functionality of the final output.


ex. of GIS map

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Scale, Area, and Page Size Relationships

Scale, area, and page size are all directly related in terms of producing a map. The scale is the factor that determines how much detail is available on the map. The larger the scale - the less detail, the smaller the scale - the more detail. A 1" = 100' scale map has a lot of detail. Parcels, street names, and building outlines could be deciphered at this scale. A map that is 1" = 2000' is a map with less detail. This map would show streets as lines and street names offset from those lines. Building outlines would be very hard to decipher at this scale; they would show up as tiny dots.

The more detail that is shown on the map the less area can be accommodated on the page. For example, if there were a map that showed parcels and building outlines at 1" = 100' at a page size of 24" by 36" then you could fit a map area that was 2,400 feet by 3,600 feet on that page. If a map was 1" = 2000' then the map would cover an area of 48,000 feet by 72,000 feet at a page size of 24" by 36".

For more information

There are many different types of geographic analysis and even more applications. If this brief introduction has triggered more interest, please follow the references on our "links" page, or contact us directly.

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