Geographic References
Geographic information contains either an explicit geographic reference, such as latitude and longitude or state plane coordinate, or an implicit reference such as a rural address, postal code, census tract name, forest stand identifier, or road name. An automated process called geocoding is used to create explicit geographic references (multiple locations) from implicit references (descriptions such as addresses). These geographic references allow you to locate features, such as wetlands, and events, such as a toxic spill, on the earth's surface for analysis.

Vector and Raster Models
Geographic information systems work with two fundamentally different types of geographic models--the "vector" model and the "raster" model. In the vector model, information about points, lines, and polygons is encoded and stored as a collection of x,y coordinates. The location of a point feature, such as a well, can be described by a single x,y coordinate. Linear features, such as roads and rivers, can be stored as a collection of point coordinates. Polygonal features, such as parcels or lakes, can be stored as a closed loop of coordinates.

     1) Input
     2) Manipulation
     3) Management
     4) Query and Analysis
     5) Visualization

Input
Before geographic data can be used in a GIS, the data must be converted into a suitable digital format. The process of converting data from paper maps into computer files is called digitizing.   Modern GIS technology can automate this process for large projects using scanning technology; smaller jobs may require some manual digitizing (using a digitizing table). Today many types of geographic data already exist in GIS-compatible formats. These data can be obtained from data suppliers and loaded directly into a GIS.

Manipulation
It is likely that data types required for a particular GIS project will need to be transformed or manipulated in some way to make them compatible with your system. For example, geographic information is available at different scales.   Before this information can be integrated, it must be transformed to the same scale (degree of detail or accuracy). This could be a temporary transformation for display purposes or a permanent one required for analysis. GIS technology offers many tools for manipulating spatial data and for weeding out unnecessary data.

Management
For small GIS projects it may be sufficient to store geographic information as simple files. However, when data volumes become large and the number of data users becomes more than a few, it is often best to use a database management system (DBMS) to help store, organize, and manage data. A DBMS is nothing more than computer software for managing a database. There are many different designs of DBMSs, but in GIS the relational design has been the most useful. In the relational design, data are stored conceptually as a collection of tables. Common fields in different tables are used to link them together. This surprisingly simple design has been widely used primarily because of its flexibility and very wide deployment in applications both within and without GIS.

Query and Analysis
Once you have a functioning GIS containing your geographic information, you can begin to ask simple questions such as:

     Who owns the land parcel on the corner?
     How far is it between two places?
     Where is land zoned for industrial use?

and analytical questions such as:

     Where are all the sites suitable for building new houses?
     What is the dominant species in this forest stand?
     If I build a new subdivision here, how will traffic be affected?

GIS provides both simple point-and-click query capabilities and sophisticated analysis tools to provide timely information to managers and analysts alike. GIS technology really comes into its own when used to analyze geographic data, to look for patterns and trends, and to undertake "what if" scenarios. Modern GIS systems have many powerful analytical tools, but two are especially important.

Proximity Analysis
     How many houses lie within 100 feet of this lake?
     What is the total number of people in a county board supervisory district?
     What wells are within 1200 feet of this proposed landfill?

To answer such questions, GIS technology uses a process called buffering to determine the proximity relationship between features.

Overlay Analysis
The integration of different data layers involves a process called overlay. At its simplest, this could be a visual operation, but analytical operations require one or more data layers to be joined physically. This overlay, or spatial join, can integrate data on soils, slope, and vegetation, or land ownership with tax assessment.

Base Maps--Include streets and highways; boundaries for census, postal, and political areas; rivers and lakes; parks and landmarks; place names; and USGS raster maps.

Demographic Maps and Data--Include data related to census, consumer products, financial services, health care, real estate, telecommunications, emergency preparedness, crime, business establishments, and transportation.

Environmental Maps and Data--Include data related to the environment, floodplains, environmental risk, satellite imagery, topography, and natural resources.

General Reference Maps--World and country maps and data that can be a foundation for your database.

Improve Organizational Integration
Many organizations that have implemented a GIS have found that one of its main benefits is improved management of their own organization and resources.   Because GIS has the ability to link data sets together by geography, it facilitates interdepartmental information sharing and communication. By creating a shared database, one department can benefit from the work of another - data can be collected once and used many times. As communication increases among individuals and departments, redundancy is reduced, productivity is enhanced, and overall organizational efficiency is improved. Thus, a county government’s databases can be integrated so that when there is planned development, affected citizens can be sent a computer-generated letter.

Make Better Decisions
The old adage "better information leads to better decisions" is as true for GIS as it is for other information systems. A GIS, however, is not an automated decision making system but a tool to query, analyze, and map data in support of the decision making process. Information can be presented succinctly and clearly in the form of a map and accompanying report, allowing decision makers to focus on the real issues rather than trying to understand the data. Because GIS products can be produced quickly, multiple scenarios can be evaluated efficiently and effectively.

Making Maps
Maps have a special place in GIS. The process of making maps with GIS is much more flexible than are traditional manual or automated cartographic approaches. It begins with database creation. Existing paper maps can be digitized and computer-compatible information can be translated into the GIS.   The GIS-based cartographic database can be both continuous and scale free.   Map products can then be created centered on any location, at any scale, and show selected information symbolized effectively to highlight specific characteristics.