GIS

A GIS is mapping software that links information about where things are with information about what things are like. Unlike with a paper map, where "what you see is what you get," a GIS map can combine many layers of information. Like paper maps, a digital map created by GIS will have dots, or points, that represent features on the map such as cities; lines that represent features such as roads; and small areas that represent features such as lakes. The difference is that this information comes from a database and is shown only if the user chooses to show it. The database stores where the point is located, how long the road is, and even how many square miles a lake occupies.

Each piece of information in the map sits on a layer, and the users turn on or off the layers according to their needs. One layer could be made up of all the roads in an area. Another could represent all the lakes in the same area. Yet another could represent all the cities.

GIS can be used by government agencies and businesses to create layered tactical and informational maps, depending on what information is needed. These data layers include water features, flood risk zones, digital orthophotos, zoning designations—even the locations of community disaster volunteers.

Why is this layering so important? The power of a GIS over paper maps is your ability to select the information you need to see according to what goal you are trying to achieve. A business person trying to map customers in a particular city will want to see very different information than a water engineer who wants to see the water pipelines for the same city. Both may start with a common map—a street and neighborhood map of the city—but the information they add to that map will differ.

Some of the primary directives of GIS are:

1. Mapping Where Things Are
2. Mapping Quantities
3. Mapping Densities
4. Finding What's Inside
5. Finding What's Nearby
6. Mapping Change
   

Mapping where things are lets you find places that have the features you are looking for and to see where to take action.

People map quantities, such as where the most and least are, to find places that meet their criteria and take action, or to see the relationships between places. This gives an additional level of information beyond simply mapping the locations of features.

While you can see concentrations by simply mapping the locations of features, in areas with many features it may be difficult to see which areas have a higher concentration than others. A density map lets you measure the number of features using a uniform areal unit, such as acres or square miles, so you can clearly see the distribution.

Mapping density is especially useful when mapping areas, such as census tracts or counties, which vary greatly in size. On maps showing the number of people per census tract, the larger tracts might have more people than smaller ones. But some smaller tracts might have more people per square mile—a higher density.

GIS is also used to monitor what is happening and to take specific action by mapping what is inside a specific area. For example, a district attorney would monitor drug-related arrests to find out if an arrest is within 1,000 feet of a school—if so, stiffer penalties apply.

GIS can be used to find out what is occurring within a set distance of a feature by mapping what is nearby. For example, a city planner may need to notify all residents within 500 feet of a proposed liquor store.
Map the change in an area to anticipate future conditions, decide on a course of action, or to evaluate the results of an action or policy.

A full GIS, or geographic information system, requires:

Hardware (computers and peripherals)

1. Software
2. Data
3. People
4. Training

and sound analysis methods for interpreting the results generated by the GIS.

GIS analysis is begun by figuring out what information is needed. This is most often in the form of a question:

• Where were most of the burglaries last month?
• How much forest is in each watershed?
• Which parcels are within 500 feet of this liquor store?

Analysts must be as specific as possible about the question that is to be answered. This will help decide how to approach the analysis, which method to use, and how to present the results.
The ordered steps in GIS analysis are:

1. Select Your Data
2. Choose an Analysis Method
3. Process the Data
4. Look at the Results

REFERENCES

GIS.com

ESRI.com

GISLounge.com

About.com, http://geography.about.com/library/weekly/aa080397.htm